If you are looking for a keyboard synth, you could get anything from Yamaha PSR series to Korg Triton. Be specific on your price range, so we could help you better.
Korg X5D is a nice one to start with and produce great sounding mix, but it costs twice as your budget, if you are looking for $300 keyboard, you can look for in ebay for same kind of keyboards, or you could get a controller and a nice soundmodule like Roland JV1010 on ebay.
Hmmm, I'm not sure about it's price in your country, but I bought mine back in 1999 only ~$400 new. I assume it must be on sale somewhere cheaper specialy for used one, right? And in addition, you'll save bucks for PC MIDI interface since X5D has also serial PC/MIDI port on the board. All you need to do is buying additional dedicated connection cable (Korg AG-001) for ten bucks or so (mine came bundled with it)