basically you will need 35 profile tyres if you get 18" rims to keep it the same rolling diametre as stock
so personally i'd go with a 245-265 width tyre, 35profile to keep speedometre correct and 18" because they are large enough to look good.. and not get stupid with tyre prices, (allthough still expensive, dont get me wrong).. but also guard bashing or lip rolling... my mate had 19"s on his 33.. and he had trouble turning!