If you’re installing this yourself, my advice is to not try to install everything at once. Install one type of device at a time – meaning install hardware, set up / modify your smartapps, and then wait a few days to see if you / your family likes the way the system works. Only then go onto another set of devices.
Do the siren last.
For thermostats, I am using Ecobee’s Smart SI. http://goo.gl/4KdQ7A
They work great with ST and there is plenty of 3rd party support and apps. The ecobee 3 is pretty nice, but I needed two thermostats and I got them on sale for $140. Can’t beat that.
Do the ST multis act as motion sensors as well? for example if I place them on the entry door, can they detect motion around it? How far does the motion detection work?
They detect acceleration, not motion. example - if someone pounded on the door - knocking doesn’t trigger mine.