I think dark gunmetal wheels are the best for most colors of cars. But on space gray, that matches too well and I think black wheels are the best color for gray cars. The key is to get some contrast IMO. Gunmetal is just right for every color except gray.
I'd go for the 19" M3 359 wheels or the Avant Garde Replicas as I have. They look stunning with the satin black finish, they only problem is they can look dull after a few days of brake dust accumulation and are a biotch to wash. They will go great with the M3 front end you have.
The tips look great! I think a CF performance spoiler would look great back there, painted or unpainted..
M3 Edition Grilles|Painted Reflectors|Matte Black Vinyl Window Trim|M3 Lip Spoiler|35% Tint|19" Satin Black Avant Garde M359|Lux H8 V3
OEM Aero Lip|M3 Side Skirts|LCI Tails|M-tech Rear|Duke Dynamics CF Performance|Cyba 4.0 Black Chrome