I recently purchased my 2000 Platnum GS400 in NY area, and I am very happy with it. It has 28K miles on it. I paid $28500 for it. The car vibrated a little at 55-80 mph, but re-balance fixed the problem.
Before buying this car, I have looked at few others of same model. Besure that all the body serial plates are still in place and match with the vehicle serial number. Check for any paint chip on the hood of the car and the bumper. Lexus factory paint should be very thick. Chipping could be the sign of repaint... Look for any damage and/or bend on the radiator as it may have front-end damage. Look for any paint spray that may get under the hood and around the doors => sign of repaint. Make sure that the engine is smooth and has good acceleration. Lexus should be very quiet on the highway, except wind noise. Look under the car for any sign of silicon spray near the fender. Dealer tends to hide fender problem with silicon spray. Test drive it on the local and highway. Look for noise from the brakes, engine belts...
In general, do a complete check of the car. You are spending 30K for it. Good luck.
Hung