You can do everything that you described with the nonADT models, or with using the nonADT model devices with the ADT model hub. It’s just that there still that cloud dependency, so if you lose touch with the smartthings cloud, some rules will still work, but a lot of rules will not. For many people, that still acceptable for an inexpensive self monitoring system. For those who want a more rigorous security system, it’s not.
But you can get all the features from the home automation side and still have a baseline security system that works even without the Internet on the security side if you get the ADT panel model. So It just depends on what your own requirements are.
One of the most significant differences is that if you don’t have the ADT panel model, there is literally no way to arm or disarm the system unless you have contact with the smartthings cloud.
With the ADT model you can arm or disarm it either from the panel or with the key fob.
That doesn’t seem like a big deal until the day comes when the cloud is out, you are away from home, you arrive home, and all your sirens started going off. 
Or there’s a big thunderstorm, your power goes out in the evening, you want to go to sleep and then you realize there’s no way to arm your security system.
But if you get the ADT model, you don’t have to replace all of your sensors. Only the ones that you want to use to Trigger those special security functions.
You can still use your non-ADT sensors and have vocal alerts or sirens go off or get push notifications if a door is left unlocked or have a leak sensor trigger a water valve or any of that stuff. As long as your system has a connection to the smartthings cloud.
You could also still use your non-ADT sensors for all your regular home automation rules like having lights come on when the door is opened after dark or whatever.
So think of the ADT security piece as an add on layer to the existing home automation system. You only have to use those devices for events that you want to have called the ADT monitoring center. Or that you want to have work when you don’t have access to the smartthings cloud.
See the following FAQ (the topic title is a clickable link)