18's seem to be the most favourable choice as they still look big, the wheel and tyre packages are well priced, the handling is not drastically affected and the pot holes don't cause too much of a problem.5x120 pcd means the stud spacing. You want 5x120mm not 5x100mm unless your looking for wheels for a golf.
e36's can run up to 20" on stock arches with the right widths/offsets but as a general rule anything above 18 is pushing it.
Ideally you want to be looking at 17/18", width of 7.5j to 8.5j and an offset of somewhere between et35 and et45. Those sorts of sizes should see you with ok fitment, no rubbing.
5/6/7/8 series wheels dont fit without arch modification, neither do e30 m3 wheels.
e46 wheels will fit your e46 but in some rare cases the offset may be so high that you need a spacer on the front to prevent rubbing on the suspension.
z3 wheels (not z3m), 1 series wheels and new 3 series wheels are all good afaik.