It sounds like a pretty good deal to me. I bought an '02 LS430UL in July (Silver, 72K) for $24,000. The condition sounds very similiar to yours. You have a great automobile that you will love for years to come. 90K is nothing for an LS430. Just spend the extra money and follow the recommendation of your Lexus dealer on the service. It will cost you a bit more up front, but in the long run you will be better off.
Are you happy with your Mark Levinson sound system? My subwoofer was rattling and my local Lexus dealer wanted $500 just to replace the sub. I took it to a local high end audio shop and ended up dopping about 10K on what has turned out to be a really awesome upgrade complete with video throughout the car, a new head unit with pop up monitor customly installed where the factory disc changer was behind the woodgrain panel, bluetooth, iPod interface, new nav system, XM radio, back-up camera and TV tuner. The best part is there is no obvious modification to the vehicle and the factory unit still works just like it did with exception of the cd changer ( but since I have an iPod interface to my new unit, I would never use the disc changer). Everything is still factory to the eye. I will post some photos in the near future. I think this turned out really great and I haven't seen anything like it before on an LS430. The sound quality is off the chart. The Mark Levinson sounded great for the average listener (typical grandma/grandpa that usually own these cars), but what I have in this car now would satisfy even the most sphisticated audio enthusiast. I'll post photos soon.