You could probably get away with 18x8.5 wheels with 245/40/18 tires on a sedan, but NOT on a wagon. Like KC1 said, there are SO many things you'd have to change to get the wagon to the same specs as a sedan. You'd have to get custom fender flares in the rear (unless there are some wider aftermarket fenders for the wagon? Never heard of any), so the car would have to spend some time in a body shop, and it'll cost $$$. You also do NOT want differnet size tires front and rear. Keep them all the same size, otherwise you'll ruin the center diff.
I have 225/45/17 Falken Azenis tires and they stick out past the fender in the front, and are just about flush in the rear. I don't have any rubbing yet, but I'm afraid that when I get my Tein Wagon's, and lower the car, I might have problems. Will have to wait and see.