Offset means from the wheel hub to the inside of your rim if you get 18"X8.5 regardless of the off set is too wide with tires when you turn you gonna rub the inner and outer fender not good at all.
Far as the offset not sure also you got a 318i you have a smaller Power steering pump 18"X8.5 is too wide and too heavy you're gonna kill your power steering pump. Go with 17" and check the offset before you buy any thing. Here's a pic of my from wheels exchange. (M5 17")