8.5" is a width not a diameter.
245/40/18 fits the M3. It's the stock size in the front (on 18 x 8.5" wheels).
The ZCP brakes are no different than the NON-ZCP cars. 18" wheels are available OEM and fit the M3 if they are the correct offset / width.
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports