Hi,
Sold my 1993 LS400 for about £5,500 with 120,000 miles on it.
Nothing wrong with it at the time - I just wanted one with Sat Nav, so bought a year 2000 model to replace it with.
The guy who bought my old one has had it about 18 months now and still phones to say how fantastic it is etc.
I would think that as long as you view the service history, and check that cam belt has been changed when it should, then you won't have any engine probs. Thing to remember is these are big and heavy cars and like all luxo automatics they can be a bit heavy on things like suspension, bushes, tyres and brake components. So, don't worry too much about the drivedrain, concentrate on the brakes and suspension as these are just as potentially expensive as parts on engines etc.
However, compared to a similar performance and specification Jag/Merc/BMW, then I think the LS400 of similar age would be in better condition and cost less to run.