That would work. It wont take much to heat it if it is well insulated. You need to have a dehumidifier for the summer though, you dont want mold growing everywhere
A large in wall AC unit would probably do the trick
i dont think i can for a wihle but is an old milk house with concret walls it is like 10x10 feet and it is built into the ground(in to the side of a hill)
I'm not sure, because i've never done it just to be sure you could put some studs in, fill those gaps with fiberglass insulation, and then put foam over that. I would be more worried about the wall that is exposed the outside.