I wouldnt get anything smaller than 7.5, unless they were 15" wheels. You wont be able to fit anything wider than 215 on a 17x7.
Get 17x7.5 in the front and 17x8 in the back
18s are just not a good idea
AND MAKE SURE THE OFFSET IS < 38
otherwise your wheels will not be flush, they will be sunken into the wells. Plus, 40 offsets are for FWD vehicles.