Turning Basement/Garage Into Room

Currently I'm renting a small house, the first floor is a 18' x 35' garage / open room. There is a lot of space being used for storage, so I want to move everything to the attic and make a room down there for my son. About 20' of it is taken up by the car, washer, drier, refrigerator, ect. That leaves me with a ~ 15' x 15' space. Considering that this is an apartment, and I don't have much to spend, I can't put up drywall. I guess I'll have to put up a curtain, but I can't find any 18' poles to hang one. There is also no AC down there, so it can get pretty dang hot. There is only one window in that little area, but it's wierd. It opens up butterfly style as opposed to a normal sliding window. I don't think I can put a window unit there, and portables are $250+ So in a nutshell, I need some advice from a professionals / hobbyist. How could I divide the room and cool it? My budget is only about $350, absolute max $500.