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 SSII M3 Coupe - KW Clubsport 3-way, Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Advan TC3 18x9.5 & 18x10.5 - Hankook RS3 (Track), HRE R40 - 18x10 & 18x11 - Michelin PSS (Street)