In my chibi experience with narrow keyboards, I have immediately found two-octave to be intensely frustrating. But I think three would be an okay minimum. I don't need 88 keys, but I do want at least three octaves.

I'd like weighted keys (I dislike the standard bouncy lightweight low-end keyboard keys that you find on the usual, say, "250 voices!" home unit at Costco) but I think I'd trade that for aftertouch. I have a fairly strong feeling I'm going to want that eventually and if by some freakish opportunity I can get one for my budget now, then I'd take that opportunity.