No diesel IS200 (there's a diesel IS220d- tbh that's a bit of a dog).
My manual IS200 does 23-26 mpg locally, but I can get between 33-36 mpg on a motorway run (@ 70 mph)
Have you considered a Audi A4 avant or BMW 320d estate? If you must have a Jap diesel I'd go for a Honda Accord CDTI tourer (my brother got the saloon version, its a very good car).