Ashley do you really want to drive on MT all the time? I would (and did) buy 17inch BMW rims for next to nothing. Use the much bigger range of MT rubber and leave road rubber on the 18s.
If your really in need of MT all the time fine but if not the noise hit and low on road traction read as safety make me put the 17 MTs on when needed. It only takes 30 -40 minutes to change 4 wheels for me and less if I can get the kids to help:)

