Only the fronts IIRC. Ie need 4 fronts unless you wanna mod the arches.

330ci rims go straight on mind.

Wouldnt have thought 18s affect the handling really at all, and do look ok.

20s just kill the handling, and need big mods unless you want gay skinny fuckers.

Not sure 19s would do it much good either.

Big wide low offset 17s are the way to go, tall skinny wheels are for homos.