Well, you're not going to get a really good piano sound from an older Korg unless you sample one yourself. Yamahas, I'm not sure; the MO6 piano sounded better than the Korg TR, which is as good or better than the Triton or Trinity (the rack versions should sound the same as the keyboard versions).
Basically, it comes down to how much you want to spend. If you can get a really good deal on the Triton Rack, that and the KeyStation 88es or another controller would be good. If you want an actual keyboard, the less expensive models you'll find in stores now are the Yamaha M06 or M08 (the Motif ES sound with less features), the Korg TR (previous generation; discontinued but still in stores) or M50 (current generation; costs more than the TR but the sound will be better), or one of the Roland Juno series.
THe big question is whether you want sampling or the capability to add custom sounds. If so, the Korg TR or maybe the Triton Rack (I know the Triton had sampling; not sure of the rack) is your best bet. The KeyStation doesn't have aftertouch (the keyboard responding to the pressure you put on the keys after the initial touch) and the TR does; out of the two, I'd recommend the TR. If you just want to add other sounds people have made but don't want to create your own, you can skip the sampling upgrade for the TR and just add the extra RAM you'll need.
If you don't want sampling and just want to use the on-board sounds (or create nw sounds using the on-board waveform data), you'll possibly want the Yamaha M06; it's better acoustically (in my opinion) than the TR or Korg M50. Ideally, you should try them both and see what sounds better to you; keyboards are all about personal preference.
That's it for what's in stores, but you may be able to find something used that might be better (I'd take the Triton Extreme with the MOSS expansin over any Korg model today, other than the OASYS, even though it's older, and I'd take a MOTIF/MOTIF ES/MOTIF XS rack and a good controller over the M06/M08, Korg M50, or Korg TR if I didn't need sampling). If craigslist or something similar operates in your city, give it a try; you could end up with something better than you could otherwise afford.