Hi, what's your budget? There are several good synths second hand on the market right now. If I may advise you, you can consider the following for < $ 500,00:
E-MU PK 6: 32mb of real instruments and a lot of very good beats. Roland XP-30: lots of good instruments and expandable with optinal cards.
KORG N5/X3: older instruments but still very vertasile and good sounding. Yamaha CS1x or CS2x: dance orientated but good.