My mkIV now on 143,000 miles, puchased at 54k as a three year old. I'm v pleased with this faithful workhorse and hope you enjoy your prospective new car as much as i have mine.
you shouldnt have any problems at all with bulletproof engine and gearbox, provided (as other posts have said) the car has been main dealer serviced. Lexus UK dealers say they have never needed to change a LS400 autobox. Main dealer service is v good but expensive. Must ensure that cambelt service has been done at specified times. When you're in for next service, ask dealer to check / clean out throttle body which sometimes accumulates carbon deposits and also check no slack on throttle cable.
My car is driven hard but surprisingly tyres last 40,000 miles a set. Am presently using Yokohama AVS dB550 as original Dunlop D8s are firstly ancient technology and secondly difficult to obtain. Best stick to Japanese tyres, I've used Contis and Pirelli P7 before (latter produce loads of road noise) and just dont suit the way this car drives.
I suppose we have to expect some wear and tear on cars of this mileage. Black interior keeps its appearance (IMHO) much better than the lighter Lexus colours. I had some problems with drivers seat sagging, so changed seat backrest and cushion, which was expensive. Also I had to change one front suspension arm at around £450 due to the bushes wearing out. Nowt wrong with the arm, just bushes - I think Maneesh put a posting some while back where says where you can get these at a good price. I have had to change TWO 'fly-by-wire' throttle position sensors, one FOC under warranty - jerky / slow response is the giveaway.
Check which sat-nav disc you have. It can be updated at a cost of around £160. My wife bought a "tom-tom go" stand-alone satnav unit to for her Audi today but my seven year old Lexus unit is way better.
original polished 16-inch alloy wheels look v nice when new but the lacquer comes off and the wheel corrodes. So budget for about £80 a wheel for a body shop to re-finish these for you. Also budget for respraying the bumpers and mirrors; the paint on plastic trim doesnt last so well. The main body paint finish should still be perfect.
Apart from the problem with the throttle sensor (I believe Lexus have changed many of these) I have never heard of any electrical problems on a LS400 - all the same, best check all electrical toys work, like the electic heated seats, radio, sat nav, instruments, xenon headlamps because otherwise they will be expensive to fix.
I sometimes think of changing to Merc or BMW 7 next time but all the owners i talk to talk about problems with these cars. So i'll stick with my present car till Lexus bring out a hybrid LS, with nice firm seats.
A final word on longevity - at my local main dealer one customer has a still fresh-looking 300,000+ miles LS400 1992 model which comes in for regular service....