I own GS450h SE, 2006 model, 109 000 miles. This is my 7th Lexus and they have all been reliable and pleasure to drive.
MK3 GS300 is an amazing car. If you can find one SE-L specs, Lexus service history and reasonable mileage then it only has one big problem: V8 powered GS430/460 :) The latter has almost identical fuel consumption, but much better performance.
450h should not be overlooked too, but again i would only recommend buying full dealer history clean example from 2008+. Do not go for the cheapest. There are many bits in the 450h drive train that are quite expensive to fix especially if you are not very good with electrics/diy.
Remember MK3 GS is getting old now, but if your budget is limited to this then it's probably the best used car you can get.
Take it for a long test drive at least 30 minutes and check everything: gearbox, engine, power steering, suspension, brakes, air conditioning, sat nav etc. Common problem on MK3 GS is squeaky noises coming from dashboard/glovebox area. Very annoying problem and in most cases time consuming to sort out.
Good luck hunting :)