Hey,
Great car, I had the sport version of this early GS. Unfortunately she won't be worth to much in today's market... Plus fuel wise you already know she'll be relatively thirsty (main reason for low value). Personally I'd look to pay around the £500 mark esp as this one isn't in top condition...
My best advice is to check the radiator. The one I had i bought from a non lexus dealer and i checked everything except the radiator. It went after 3 months of ownership and the car was history!
Let me know how you get on :)
B x