We do need to know which region you are in, as the device selection does vary somewhat.
If you are in the US, personally the only smartthings model line that I would recommend for security is the ADT/SmartThings security panel. And indeed if you follow any of the links on the SmartThings site to “security” it will take you to that model line.
That’s because it’s the only model which does not require both Internet and the smartthings cloud in order to operate the security functions. You still get all the same home automation options. But the ADT/Smartthings Model is the only one with cellular connectivity (rather than using the Internet for all of its notifications) and is also the only model which can be armed or disarmed if the smartthings cloud is not available.
When considering security options, and in particular when comparing it against competitors like Abode, it’s very important to understand that SmartThings is still primarily a cloud-based system. So even things like Camera integration only work when the smartthings cloud is available.
And there has been an outage of some kind at least once a month for each of the last 20 months except, I think, January 2017. Plus they push out hub updates pretty often, usually at least once a month and sometimes more frequently, and while the hub is updating your system is not available. Usually that’s only a few minutes, but sometimes it’s several hours. And these updates can be neither delayed nor denied. That’s very different than most purpose-built security systems.
There are no official Camera integrations except for Arlo and the Ring Video Doorbell, even for the ADT model. And it’s not really a camera oriented system: it’s not like live feeds pop up when there’s an alert or anything like that. You’ll still probably have to use the cameras own app for that stuff.
I think the ADT model is a decent low-cost DIY security system, and certainly worth considering. But most of us with other smartthings hubs use a different system for security. Smartthings was built as a cloud-based home automation system so that they could continually add new devices. Which is very cool, and it certainly excels at that. Things are changing all the time, and there are frequent updates. But that’s not necessarily what you want for a security system.
As far as whether you need door sensors, motion sensors, how many of each, etc. it depends very much on the layout of your home, how many people live there, and how serious you are trying to get.
If you live in a high crime neighborhood, are concerned about home invasion, and have a four bedroom home, then none of the low-cost systems are likely to meet your needs.
If you live in a studio apartment in a low crime neighborhood, are the only one who lives there, don’t have pets, and are concerned about daytime petty theft break-ins (or even landlord intrusions when no one is home), then any of the low-cost DIY security systems will probably work for you with just a doorbell camera, maybe a living room camera, and a motion sensor for each room with an exit to the Outside.
But there are just a lot of different factors that go into deciding what you will need, as I know you have discovered.
(BTW, i’ve moved this to “projects” so you can get individualized responses based on your own needs, preferences, and budget. Different people will need very different things.)