coupe 18s and 17s have staggered tires but the rims itself is the same exact size. 18x8.. im not sure on the width of 17's tho

 

When it comes to winter tires, narrower is better since you'll have less hydro(snow)planing. I know mine's an '04, but the stock tire diameters (comparing 18" and 19") are pretty much the same as your 06, and here's what I've been running the following on my coupe for 4+ winters:

225/50-17 front Dunlop Winter Sport M3
225/55-17 rear Dunlop Winter Sport M3 (just switched to the Winter Sport 3D for the rear this season)

on ASA JH3 17x7.5" wheels all around. These fit over my Brembo brakes with no problems, and I have no issues with speedometer, alignment, etc. I love the Dunlops in the winter.

 

I also have an 06 coupe with the sport tuned suspension and the 19's. It also has the Brembo brakes.

Will the 18-inch wheels be big enough to give enough room for the Brembo brakes?

Will the 17's?

 

06 dont come with brembo brakes but yes they will fit