To run 265's square, you will want to run the 18x9.5's, not 18x9's. The 18x9's are not the optimal wheel width for a 265, and they are the wrong offset front and rear to just bolt directly on the car.
The ARC-8 is available in two offsets. ET35 and ET22. The ET35 offset is perfect for the front on cars without a BBK. The ET22 is perfect for the rear. You could potentially run the 35's up front and the 22's in the rear, but then you can't rotate. You are better off running the ET35 wheel front and rear. A spacer is not required in the rear, many members are currently running that wheel/tire size in the rear on Z4m's without a spacer, but I would personally run a 12mm spacer in the rear to correct the offset. A lowered car will not have rear rubbing issues as both wheels are more conservative than the 18x10 that members run with the 275's.
The photo below is off a stock Z4M with 18x9.5" ET35 wheels in the rear and 265/35/18 starspecs mounted. You can see it fits fine, but technically there is room to run a more aggressive offset like the ET22.
If you are running a BBK up front like a stoptech 355, then because of the spoke concavity, there is only 0.5-1mm of clearance on the very edge of the caliper. A 3mm spacer would be recommended for spoke clearance. 3mm spacers will keep the car 100% hub-centric so there is zero downside to running them.
The photo below is of a Z4M with 265/35/18 pilot cups on an 18x9.5" ET35 wheel and negative camber dialed in. I do believe a 5mm spacer is being used in this photo up front, so if you don't have a BBK, then less camber is needed to keep the wheel under the fender. I don't know if the camber on this car was achieved with camber plates or shimming out the strut. Plates would be the best way.
I can see in this photo that the same sized rear wheel is used (et35), but I cannot tell if it's been spacer'ed out at all, as it appears to be more aggressive then the black car above. My guess is it has.
The final setup of wheel/tire size and camber settings is basically identical to what E46 M3 owners run as an optimal track setup. They have more front fender clearance, but if you don't run a decent amount of front camber you will be slow.
Camber plates are highly recommended for any car that sees more than 2-3 days of track use a year, and definitely if you have a dedicated set of track wheels, as that allows you to adjust your camber from street to track mode in just a few minutes.