Problem I have is that I don't see a very low mileage Ls400 be any better than one with 150k on the clock. Maybe that's the case with other old cars, but not with the Ls400. Hence I don't see why it has to be 5 or 6 times more expensive.
I've had a 45k 1993 Ls and a 215k also 1993 - I couldn't tell the difference in day to day driving.....
Ok so maybe you need to replace the UCA's or a few ball joints on the higher mileage car, but you can buy a hell of a lot of suspension parts for £10k :)