Yamaha Home Keyboard?
I don't know if these are sitting around any professional studios, but they sound pretty good to me...I have tried to help a friend find a keyboard before and I came up with some minimal requirements (from my veiwpoint, anyway) for an acceptable keyboard. Beside being fairly modern and sounding good, I think a keyboard should at least have "touch response" that is, when you strike the keys softly you get a corresponding soft sound, when you strike the keys hard you get a hard sound (like you would on a real piano). Polyphony should be at least 24 (probably 32), Polyphony is the total number of notes that can be sounded at one time (a three note chord would use up 3 notes of polyphony)...although some entry level keyboards have 12, if you get into MIDI and sequencing (and layering sounds) you will need the extra polyphony. There are many new and used Casio and Yamaha keyboards with these features that can be had for under $200 (on ebay for instance). The Casios that I have heard are all right, but I prefer the sound of Yamahas...I would check anything by Yamaha with touch response and 32 note (or voice) polyphony made within the last 4 years. I am looking in a Musician Friends catalog right now (musiciansfriends.com) at a Yamaha PSR292 for $199 (you would also probably want the "survival kit" for $30, that includes a power adapter)...If money is really tight you might find a similar model (a few year old) on ebay for $100-$120. If at all possible, of course, you want to listen to a keyboard before you buy it. You might find these Yamah Home (or PSR) type keyboards in some music stores or even Walmart, or you might ask around and find one you could try out and get an idea how these keyboards sound (although there are many models many seem to have the same "sound engine" or sound source)
As far as keys: I think 61 (keys) is probably the most popular keyboard, even in the professional world (look through a catalog or music store website)...If you need to play higher or lower notes you can usually move the octaves up or down on a 61 key keyboard to accomodate this...Maybe if a person is a true pianist and uses the breadth of 88 keys in one song, maybe then they would really crave the extra keys...