All Lexus inline six and V8 cars are virtually bulletproof. Like all high-end cars you want to find one with complete dealer service records (any dealer can check for you if you supply the VIN to them).
The GS400 is a real performer in a slick and comfortable package.
I still prefer the German cars more on emotion than on content, but I still feel the Jap GT's are almost "too good". They shift too smoothly, they are too quiet, they have a bit too much chassis isolation for my tastes. But the fact remains that they hold all the records for buyer satisfaction and reliability (as long as you don't include the ES250/ES300).
Sounds like Ambush has you covered on dealer connections, and you probably will be happier in the long run buying from a Lexus dealer cause you will make friends with them easier and will have some pull when it comes to service scheduling and loaner cars, etc.. (but you will certainly save money buying from an independent if that matters to you).