Pretty sure that a trader (as opposed to a private seller) is legally obliged to give you some sort of warranty. I would be wary of any seller where it doesn't "feel" right and just walk away. Don't worry, there are loads of used IS200s out there, and it's a buyer's market, so you'll find an even better one soon.
Make sure you check whether the cam belt has been changed on any car, the interval is 6 years or 6,000 miles. The service book will say 10 years/10,000 miles but Lexus changed that. There might be a sticker on the engine saying when it's been changed, or you can see if there's a receipt, or you can get someone to take off the cam belt cover and see how new/used the belt looks.