As this is the newbie section, no question should be too simple to ask.
There are two companies that make ATCS. Stoker and GURU. GURU was the first one and the ATCS section was just called GURU. Stoker came along and add many bells and whistles, multiple cooker control, connect to a computer, display on web, etc.
All of them control the temperature the same way. Monitor pit - Low temperature, blow air. Correct temperature no air. If you q'er has air leaks, you will not be able to keep your temperature down.
The simplest device is the NanoQ from GURU. Monitor the pit temperature and keep it at your set temperature.
Most sophisticated is the Stoker with multiple ports. Control 4 cookers and the meat temperature at the same time.
"preferred brand of these units"
In the ATCS section you will find mostly posts on the Stoker. As you can do many more things with it, you will get more questions. I am a GURU owner from before the Stoker came out. So that's my preference. What other people prefer shouldn't make your decision, what you want the ATCS to do should be how you decide.
"the price range"
You can get a basic GURU for $189. Pit controller, no meat temperature.
A Stoker Dual Cooker with 2 meat probes is $420.
"level of computer expertise needed to install"
No computer experience necessary. Computer expertise is only needed if you want to track you pit's progress or view your temperatures via the web.
"where to get them? "
BBQ Guru -
BBQ Guru
Stoker -
Rocks BBQ
Now go out there and browse the ATCS section -
ATCS