If you need piano & bass, and you only want to spend $500 or so, you'll probably just need to scope out what you can find with digital keyboards. If you look around, you might find a Yamaha DX7, Korg M1, or Roland D-50 out there. (But you'll have to look around on ebay or your local used musician stores) There are also rackmount boxes that have all the sounds inside (but you do need some sort of keyboard controller with midi to coax the sounds out of them). You might be able to score a Yamaha rm1x groovebox sequencer/soundsource and still have enough to get a used keyboard of some sort. If you're not sure what to do, just ask around, check out what's out there, and spend wisely. You can be pretty sure there's nothing out there new that's in your price range, (unless you want Casio), so either save up some more for new gear, or see what you can find in the used market. Good luck, but if you know how to work it right, you can find some great sounds out there.