What is the largest size wheel and tyre i can fit to my E30 BMW without having to modify anything?

I want to fit some alloy wheels to my E30 BMW 316 and want to go as large as possible without having to modify the arches. The suspension i not lowered and everything else is standard. I would like to fit maybe 17 or 18" rims if poss. can anybody tell me if they will fit with no problems?