I agree with jellybean. I recently bought my first LS400 and can't imagine what I could replace it with apart from another one. It's an amazing car, and so relaxing to drive that after 3-4 hours on the road I don't feel tired.
One of the problems when buying one is that cars over 10 years old don't have a fixed market value as such, like 2-3 year old ones do, so to some extent an LS400 is only worth what someone will pay for it. Having said that, the one you're interested in sounds like the best you're likely to find - it's from the last year of manufacture and has a very low mileage. On that basis, and assuming that you're prepared to maintain it properly, you should be able to run it for 10 years. It probably cost around £50,000 new, so you'd be getting a lot of car that someone else has lost most of the value of, and over the next few years you won't lose much more in depreciation. When you take that into account, it shouldn't cost much to run - that's what I'm hoping about mine, anyway. I only use it for long journeys (I have a smart car for running around town), when I just set the cruise control to 70 mph and get around 30 mpg.
With the advice of other members on here, I drew up a checklist of things to look for when I was Lexus hunting, and I'm happy to send this to you if it's of use. Good luck with your search.