Are you buying tires or wheels or both? One tire/wheel as a replacement or two or a whole set of 4?
If you're replacing just one wheel don't mix an 18x8" wheel with 18x8.5".
Beyond that, need more info to give any useful guidance. Though I'll mention I don't see any good reason to go narrower than the stock 8.5" width *except* for running snow tires on a separate set of wheels for winter. 18x8" could make sense for that, though it presents a minor tire sizing dilemma.
