48 is waaaaay too much offset. You'll likely be hitting the spindles with an 8" wheel, and if you aren't it will look and handle like crap(you're effectively reducing track width with high offsets), which pretty much defeats any and all purpose of going to 18s doncha think? Somewhere between +30mm and +39mm is what you need.
To answer your second question, the wheel. But both will hit the spindle if we're now talking a 9" wheel with the same +48mm offset