Hi Martin
The GS450H is a brilliant piece of engineering. However it does have a few weak spots. When buying check the following:
Seized calipers - expecially rear ones.
Rusted exhaust back boxes.
Leaking Shock Absorbers - Lexus is the only supplier for replacements.
Creaking dash.
Headlight bulb flickering or going out after a while - Remedy is to swap the bulbs over or replace them.
Rust on the leading edge of the bonnet.
Faulty TPMS Valves
HYBRID BATTERY: As above a health check is available from Lexus for vehicles under 10 years and 140k miles. On the flip side if you can't get the health check and have battery problems: it is normally 1 or 2 cells which are low - these can be replaced and the battery reconditioned for aorund £800. A member on here Richard is recommended.
Hope this helps