Welcome to the forum by the way!
There is a thread here that might help:- https://z4-forum.com/forum/viewtopic.php?p=1623544&hilit=CSLs#p1623544
CSL rears are 9.5J X 19 ET27 so unless you have bought an M I don't think the rears will fit inside the arches - you'd better check that before you order at those prices!
Which is probably why 18 Reps are popular.