20's will NOT fit on a E36...I've done a lot of investigating and have become real personal with the guy at Discount Tire. Only a few 19's will fit and I even had a hell of a time finding 18's that will fit with the right offset and bolt pattern. I'm going on my 3rd set of wheels and tires in less than a year trying to find the right ones that fit AND look good.
I have a 96 328iC, the first set was 540 sport reproduction rimes from voxx 18x8...I kept bending them with those damn pirrelli tires, next(current) is a set of 17's 17x7 and 17x8, I then put lowered springs on and with 245's on the rear it just barely rubs and I don't like the way they look. I just received my new 18's that I had made with a custom offset by Boyd Coddington (Shyster's) and am now awaiting my car to come back home from the body shop(oops). There is a M3 that comes into my dealership with 19's on it and it looks real sharp, but those are few and far between.
If any of that helps, great...if I was just rambling on...sorry!!