Chris, with your budget I would be aiming for a 400 rather than 430, and Mk 4 is the best one to go for, but you can also pick up good Mk2 and Mk3 models where the owner has done work to get them up to scratch.
Suspension is usually the area to look at, as being a heavy car it does put a strain over 10 to 12 years on all the mechanical parts (especially given stat of UK roads). However I would rather have the ride of an old LS 400 over an old Merc or BMW, as though parts on Lexus are not cheap, they generally are no where near as bad as the German motor company(s).
I would test drive at least a MK3, the Mk4 has stability control (which was my need), and drive over some speed humps, road dips, and listen out for clunks and knocks, that indicates you will have to spend some money sorting that out in next year or two.
I had a Mk2 which clunked a lot, but was perfectly okay to drive, and had it like that for 4 years and never failed an MOT.