Yes it is possible to add a window, the biggest issue is finding the actual window. It will more than likely have to come from a salvage yard.
As to the things to check:
Normal RV appliance checkout, Make sure the AC and refer work. They are 2 of the biggest cost replacements. Drive train is much like any used car, except the fact that the rear suspension is Air Bags. These need to be checked for rot. Genset should run and be able to run the roof air. Tanks should hold liquids and not leak.
If all of those things are in Ok condition everything else can be fixed as you go. Upholstery, drapes and flooring are areas that if it is new and you like it, bonus. If you feel it is a bit worn and needing replacment you may be able to do some dealing, but almost everyoune replaces it to match their tastes.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato