Correct on the E9X M3 specs. 19s are forged, 18s are cast. Both a total PIA to clean.
I might be selling my front 18" rims from my M3 (219s). You might be able to find someone willing to split another set of 18s up on M3 post.
2013 F30 328i Melbourne Red
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red