I drove the other white E30 325i. I have had a smattering of different wheel sizes from 14,15,16,17 in wheels.
One of the best fitments for the e30 is the 16×7 wheel with a 205/50/16. This maintains the stock 24″ overall diameter of the tire as specified by BMW. As Jeff pointed out the 205/45/16 will give you a smaller diameter effectively increasing your acceleration.
For autocross I have used both 15×7 225/45/15 (Toyo R1R 140TW) and a 15×8 225/45/15 (Hankook R-S3). If you keep with the 7″ wide wheel the wheel offsets SHOULD be okay to where you won’t have to roll your fenders. With my 8″ wheels, I had to roll my rear fenders to fit the 225/45/15 tire (which is almost an inch smaller than the stock tire diameter giving me more acceleration.)
in your case finding a 225 or 215 M+S tire for a 15″ or 16″ wheel will be rather difficult. All of the ones that I saw are summer performance tires and r-comps. I would stick with the 205, a much better selection of tires, they fit better in the wheel wells than 215 and 225s.
Now I run two different wheels sets, my track set, 15×8 225/45/15 with sticky tires that comes in at 35lbs per corner.
My street setup varies with a 17×7.5 currently running 225/35/17 and also in the past and in the near future 215/40/17.
As regarding which M+S tire I can’t help you because I never put less than a Ultra high performance summer tire (as per tirerack’s description) on my car. Living in the Bay Area with a car that will never see snow or off pavement duty, it is something I don’t worry about.
Sorry if that was a little scattered feeling.