If you're going with a MIDI input, the Roland PC-200 (or whatever the present day equivalant is) is a decent 49 key controller.

For USB inputs, the M-Audio lineup is the one to go with.

I mostly use guitar synth, but for stuff like long sustaining strings I use a PC-200. Nice and small with just the right amount of 'stuff' on it.
I bought mine used off eBay for about $100 U.S.

I haven't used M-Audio keyboards, but have heard good things about them in reviews.

Just a personal observation, but I wouldn't get too swayed by all the bells and whistles on some of them, like midi mixers built in etc..
They're handy if you really need to be 'hands on' with the mixing board for a small number of tracks. But if you're mixing any more than 6 tracks at a time, they're useless.