Welcome! 
I moved this discussion to projects so you can get individualized responses based on your own needs and preferences.
Conditionals
At this point, the conditional programming is the simplest part of your request, as core is an extremely powerful and sophisticated rules engine which was created by a community member and is very popular for those with a technical background. It could be quite confusing otherwise, however, And you might need help from community members to set up individual rules if you arenāt already a programmer. You can basically think of core As a scripting language for SmartThings.
Devices
SmartThings is one of the few inexpensive home automation systems that lets customers upload their own device type handlers. And there are a number of community members who enjoy writing device type handlers and are willing to share them with others. That means thereās a lot of code available. Usually the easiest way to find it is to try the quick browse lists in the community ā created wiki:
Using someone elseās code is pretty straightforward, essentially you just copy it and paste it into your own account. The following FAQ explains the concepts:
If the device is a certified Z wave device or a certified zigbee device using The zigbee home automation (ZHA 1.2) Profile, then that code is probably all you need to make it work with SmartThings. Zwave and ZHA Devices are the most common ones in a SmartThings installation, and Z wave in particular offers a number of switch choices for residential use.
There are also some official integrations for devices like the Philips Hue Bridge, the harmony home hub, the Lutron Caseta line, LIFX, etc you can find those on the official compatibility list:
For other devices/protocols, there may be individual community members who have created an integration method, typically using at least one additional device like a raspberry pi to act as a āman in the middle,ā but these often require considerable technical skill and may not be kept up-to-date. They are interesting maker projects, and may prove to be very useful for others, but they do require a lot more research and effort to set them up and keep them running. Most of them are on the quick browse lists under project reports, and some of the more popular are also on the protocols list in the community-created wiki.
http://thingsthataresmart.wiki/index.php?title=Supported_Network_Protocols
For example, there is one for Lutron RA2, but the community member Who created it is no longer active in the community, and I donāt know if itās currently working or not. There have been quite a few platform changes since it was first released.
The insteon protocol is proprietary to their system and doesnāt work directly with SmartThings. There are some community members who previously had an Insteon system and have developed some āman in the middleā integrations, but I donāt know anybody who would use those if they didnāt already have insteon devices.
Fireplace
You didnāt say if this was a gas fireplace insert or an electric fireplace. If itās a gas fireplace, in most US jurisdictions there will be very significant limitations on how much remote control you can create for it, for safety reasons. In many places, a fireplace remote can only legally be operated within about 20 feet. So as soon as you add the fireplace switch to SmartThings, you are in code violation, and may void your homeowners insurance. It doesnāt matter whether you never intend to use the SmartThings app while you were at the office to turn on the fireplace ā ā just the fact that you can is enough to violate the code.
And itās important to note that there are occasionally reports in the forums from people who have lights mysteriously come on when they arenāt supposed to, or garage doors open, or door locks unlock.
http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports
It doesnāt happen to everyone, and it doesnāt happen all the time, but it can happen and itās enough to raise safety concerns if you donāt also build in a number of non-ā smartthings-controlled safety checks.
You can find project reports regarding fireplaces that discuss all these issues. But definitely check with your local township first.
These laws apply regardless of what Home automation system you use, itās nothing specific to SmartThings.
If itās an electrical fireplace, then there arenāt usually the same code restrictions.
HomeKit
I have HomeKit and like it and use it for most of my simple automations. But there is zero integration with SmartThings. HomeKit requires a particular hardware chip in the devices that work with it, so it canāt usually be retrofitted onto existing devices. SmartThings originally intended to be compatible with HomeKit, but, like many other home automation companies, once they found out about the hardware requirement they decided to take a wait-and-see position.
There are some individual devices that do work with both SmartThings and HomeKit, such as the Phillips hue bridge, the Lutron Caseta line, and the ecobee thermostat, but there arenāt many and not enough to cover most peopleās full uses. But if your needs are just lighting and a thermostat, you can do both. So it really comes down to a case by case evaluation of possible solutions.
Nest
Nest isā¦complicated. Officially, there is no integration, and unlikely to be, as nest requires that its partners limit the programmable access that their systems offer to the thermostat and in SmartThings thereās just no way to do that. Once youāre in, youāre in.
There is a very popular community created integration, but it is technically a violation of the terms of service of your nest account, so officially smart things says āuse it at your own risk.ā It is something that could be shut down in the future if nest chose to do so (the Chamberlain garage door opener did just that with an unofficial SmartThings integration about a year ago), but a lot of people feel they will cross that bridge when they come to it.
https://support.smartthings.com/hc/en-us/articles/204733384-Does-SmartThings-work-with-Nest-
Ecobee doesnāt have the same issue, and as just mentioned, ecobee can also work with HomeKit which nest cannot, so itās also a very popular thermostat in the community. And there are some simpler Z wave and zigbee thermostats as well for people who want to spend less money.
There is one official integration option for nest/smartthings, and thatās to use the free IFTTT service. It works fine, itās just much more limited in what you can do with it. But it would be possible to use conditionals as you described and access nest through its IFTTT channel/service.
The SmartThings IFTTT service is very popular and gives you full access to all of the devices on your SmartThings account, so it does make it easy to add control to many other devices/services, although you may not have full feature access to some of them. It just depends on what theyāve exposed through their IFTTT channel.
Sonos
Thereās a Sonos integration, but the official integration may not be as granular as you want. There are some additional community ā created integrations, but I would definitely do some reading in the forums for more details on that. But this is one of those places where you may not be able to do everything all in one app.
One app to rule them allā¦
You can get most of what you want in just one app, the official SmartThings app, with some exceptions like Sonos as mentioned above. However, you may have to do some work with the web interface as well, called the IDE ( integrated development environment) , especially if you start doing some of that cut and paste we were talking about with regard to devices which are not officially supported. That isnāt something that you have to do every day, itās part of the set up for a new device or a new smartapp. But it is just something to be aware of.
Also, while itās one app, itās not a particularly easy to use app, and some choices just arenāt available through the official features. For example, right now there is no way using only the official features to set up one scene that sets three different dimmers to three different settings. There are lots of different ways to do it, itās just not always clear how to get there, and it may require custom code. So this is one of those ābe careful what you wish forā situations. Thereās one app, but I can pretty much guarantee itās not going to be your favorite. 
Again, this is an area where the company has said they want to make improvements, but theyāre not there yet.
The Unasked Questions
Since you are new to home automation, I think what Terry was getting it in his reply is that thereās a whole set of questions that you didnāt ask having to do with reliability, stability, budget, and rates of change.
Home Automation has been around for about 30 years, but itās only been around at the under $3,000 price point for two or three.
You could definitely get everything you have on your list if you went with a high-end system like control 4, and it would be reliable and stable. But it also typically costs 10% of the cost of your home, plus an annual maintenance fee. So out of budget range for most people. If I had the money, Iād get a control 4 system for sure, but I donāt. My budget is about $200-$500 per room, and I donāt do every room in the house. And an equivalent control 4 system would probably cost me 30 or $40,000, and itās just not in my budget range. And some people have Control 4 systems that cost $100,000 and more. So, The future is now ā ā but only if you can afford it. 
At the low-cost end, there are a number of competitors, and each has pluses and minuses. The SmartThings plus is that it is so customizable, so itās very easy to add new devices and, as mentioned, has an amazing conditional rules engine in core.
The minuses are that itās just not very intuitive, and if you want to get access to all of those power features you may have to come to the forums and get help. And, it has ongoing issues with reliability. The company is very aware of those issues, and has committed to improving in that area, but theyāre just not quite there yet. For example, just this week Several people reported that devices that they hadnāt made any changes to themselves had started acting in a different way, including light switches that now have to be pressed at the bottom for on and at the top for off, the reverse of the previous.
One reason those changes can occur is that SmartThings is primarily a cloud-based system. For example, even if you just want to have a Lutron switch come on when you walk past a motion sensor, The system has to send a message over the Internet to the SmartThings cloud and get a response back before anything happens.
So that means two things. The obvious one is that if the SmartThings cloud is not available, a lot of things in your home automation system may not work.
The not so obvious one is that if the company makes a change to something in the cloud, and many changes are never documented or announced, things may start acting differently at your house. Youāll see a lot of forum posts like āis anyone else seeingā¦?ā Because whenever anything gets flaky at the house, the first question people ask is āhave they changed something?ā
Thatās not true of other non-cloud-based systems. For example, if Lutron or Phillips hue change something, youāll usually know, because youāll have been told an update is available and you get to control when you apply that update. SmartThings, however, can change code in the cloud whenever they want and that will affect your account.
For many people, thatās an inconvenience theyāre willing to live with in order to get The power and flexibility of the SmartThings system at such a low dollar cost. But it is something to be aware of. At present, SmartThings is a pretty high maintenance system, although again, the company has publicly committed to changing that. But they arenāt there yet.