GO FOR IT! If you have not driven one yet . . . try it - you will not be disappointed. I picked up a 2001 GS430 sport navigator last January and I am still in awe at the power and smoothness of this car. I had a LS400 before this and the difference is significant. I'm hooked on V8 power and nothing compares to Lexus.
As others have said, the GS430 first appeared in late 2000. The difference between the sport and the se is that the sport has a different front grille and "touring car" style front bumper, a rear spoiler with brake light, cooler looking 18" alloys, "sport" logo etched on rear windows and front spoiler and firmer, lower suspension. Internally it has darker wood trim. If you can find a navigator model it's worth paying a few quid extra. This gives you touch-screen navigation, climate control, audio and other info. It really improves the front console and adds to the wow factor when others get into the car. Hope you find a good one!