for 1k you'll want to be looking for a high miler thats been looked after and had necessary work done vs a lower miler thats junk. You should be able to find a good example bar the mileage which doesn't really matter too much on these cars especially being 6 cyl.
Ive always wanted a IS but when I was serious about buying one i spent 3 months doing research and monitoring the market. The only thing ive found from owning one that i didnt realise is the brakes can start sticking, but i still class that as wear and tear, my cars 12 years old with same calipers. Anyway when you view get the jack and check each wheel spins free. check the cambelt cover to see when the cambelt was last changed if there isnt a invoice. check the shocks are newish,
BTW the so what if the owner has lost his MOT cert. All MOT certs are logged digitally and can be viewed on the vosa website, just type the reg in, you'll soon see any advisory's.