If you're going to buy a used fork, you might as well buy a whole used bike. The technology from the nineties is getting pretty dated now. Bikes are lighter, stronger, faster, etc. You should be able to get a decent hardtail for not much more than buying a decent fork. If you want to buy a used fork, try a coil spring Marzocchi or a Fox. I have never seen a good used Rock Shox or Manitou fork. The RST might not be bad, but don't expect a high-performance ride.