What year GS are you thinking of? I'm assuming from what you have said that it would be a mk1 (92-97) and if a sport, then they were only done on the mk1 from 95-97. I've had a mk1 Sport, and now on a mk2 Sport. The mk2 is definitely a better motor, although in a lot of respects I prefer the shape of the mk1. Having said that, the longer I own the mk2, the more dated the mk1 seemed. I have always loved the Sport wheels on the mk1 (only with the 10" wide rears though), so much so that I sold the mk2 Sport wheels on my mk1 and kept and refurbed the mk1 wheels on my mk2. They suit the mk2 great as well.
I had my fair share of issues with my mk1, I seemed to have been unluckier than most. Crank oil seal went, head gasket went, front bottom trailing arms, radiator, alternator, starter motor, wheel bearing. All in all, a good few grands worth of stuff. So far on the mk2 (touch wood), no problems at all.
Seizing calipers and slide pins on the brakes are a common issue on most Lexus, easy enough to rectify. Suspension parts can be expensive, so check it all out for play. Engines are generally bullet proof, although the rear crank oil seal can be a weak point on them.
Tyres, especially with the 10" rears 275/35 18's can be expensive, but online places like camskill or blackcircle can be reasonable.
Other than that, happy hunting and welcome to LOC :)