Every system has pluses and minuses. For example, homeseer doesn’t have support for zigbee devices. So different things work for different people.
As far as code and costs, most community members are happy to share their custom code for free with other smartthings users. There are a few who do charge a license fee. In the case of locks, both Lock Manager by Erik Thayer (totally free) and RBoy’s lock offering ( behind a paywall) are very popular. You just have to look at the various features and decide what’s best for you.
The challenge for many people is in finding the custom code to begin with! Fortunately, there are quick browse lists in the community – created wiki which list both smartapps and device type handlers by type, and that’s usually the fastest way to find stuff. For example, in your case you would just look on the list for locks. Normally community members write something for themselves first, and then share it, so the features do vary.
Normally community members also have an author thread in this forum where you could ask questions or even make suggestions for additional features to any individual smartapp.
There is also a section in the quick browse list for project reports with lists on everything from pets to holiday projects. Those can also be helpful. 
In addition, there is a very powerful community – created rules engine, webcore, which is essentially a (free) scripting language for SmartThings and lets you do almost anything. If you have a strong technical background it’s worth looking at, and there are many community members who will be glad to help you if you get stuck on anything.
So at least the lock is easy to solve in SmartThings. I know there are community members who are using blue Iris. You can check the quick browse lists to see if anything jumps out at you coming otherwise I would start a new thread in the devices section and just ask how people are using blue Iris with SmartThings. I’m sure you’ll get lots of answers.
On the alarm system, that’s more complicated, and you probably will have to purchase some additional equipment. Again, check the quick browse lists and look in the project report section for security. They have been some changes over the last couple of years as far as how people do this, so I don’t know what the current options are as I myself just to have a completely separate security system.
As far as Imperihome, I’m not familiar with it, but I did notice a couple of threads in the forum about it. I don’t know if you’re already participating in those, but they look pretty active. I haven’t read any of the individual posts yet.
Sorry I can’t be more helpful on some of the specifics, hopefully other community members will chime in. But at least now you know about the quick browse lists, and they should help any future research.