Price of the IS200's are dropping like a lead ballon.
I wouldn't pay more than £1000-£1250 for the car you are looking at- service history/invoices is a must regardless of age IMHO, they tell you how a car has been treated in the past by previous owners.
IS200's don't fetch much money nowadays, I traded my 10 year old IS200 in at a Merc Dealer (for a C220cdi Sport) on the weekend- best I could get for it was £1k- that had Lexus service history up to 90,000 miles and the rest was a stack of invoices for servicing carried out at independent garages.
As said, there are better IS200's out there.