All of them are the same size and fit the same size hole (14X14). We went with a 13500 BTU Carrier Air-V Low profile Model Because i didnt want it to be so tall sitting on the front of the rig. Lots of people go with a full sized unit on the front instead of a low profile unit, there is no difference in the way they work, one is just taller than the other.
BTU? You can use anything from a 9000 BTU up to the full size 15000 BTU. Some owners who install the 15000 BTU model complain that it cycles on and off to fast and freezes up, Some of them say they dont have that problem. My experience with our 13500 BTU model is that it works great. We were at Kerr lake a few weeks ago and the outside temps were 102 to 106. With both units running, We could maintain 65 degrees in the camper and both units were cycling on and off like normal.
You can get a thermostat kit and hook it up to a wall mount thermostat, but that isnt needed. The built in thermostat works like it is supposed to. Mounting one on the wall in our camper would probably put it directly into the cold airflow from the unit and make the unit cycle off sooner.
One other thing you need to remember is that with the units designed to be mounted up front with built in t-stat, the fan runs all the time and the AC compressor cycles on and off. think of it like your car, the ac is on the fan blows all the time. Unlike your main AC unit that cycles both the fan and compressor. Some people dont like the fan running all the time, some people do. Personally I do. Even when the compressor is not running its helping circulate air through the rig.
Ordering one and putting it in yourself will be less than half what a dealer charges to order you one and install it. it has been well over a year since i did any research on these, but when i got ours I did alot of research. I ended up ordering from
http://www.adventurerv.net/. they had the best prices and awesome customer service, they dont have to order the unit, they have them in stock, quick shipping. Awesome people to work with. you can get a non ducted low profile unit for between $700 and $800, A full sized non ducted unit even cheaper. Carrier got out of the RV AC buisiness, and since that is the only one i have experience with i wont make any recomendations on which brand to buy. Adventure Rv has all of them.
Good luck, You will be glad you did this!