If you want something short for CQB and your not sold on an MP5 (acquired taste really), then look for an LR300 or G36C.
Both good carbines with folding stocks. Just don't succumb to the idea of throwing a folder on an M4 platform just because you can in this make believe universe.
I'm not sold on the idea that you need a folding stock for CQB (especially in the real world). But after gaming my LR300 (which is probably going off the chopping block I love it that much) I'm starting to take a liking to them.
Honestly your first AEG is all a learning process. You will either get real serious and buy something else (either replace or supplement the original) or you will decide this game isn't for you and resell for little loss.
Ultimately if you've come from the other shooting sports like I did before airsoft, all this stuff is pretty cheap in comparison. Especially to a high-volume pistol shooter.