Buy an M and don't look back. In my 10 years of owning a Z and being on various boards, one constant holds true. The owners of lower cars tend to start out stating they are happy with the performance and within short time, after they have acclimated to the car, always seem to want more. By getting an M, not only are you meeting most of your stated needs, you will be forestalling the urge to modify for performance all the more. In terms of price, the Z4 M roadster are an absolute steal right now. Prices are going to be falling in the next 90 days as the sunny season comes to a conclusion and folks want to sell. Get all the car you want so there are no regrets down the road.
SS braces, AST 4200's coilovers, Vorshlag camber plates, GC RSM, H&R Front sway, Conforti CAI, Magnaflow exhaust, Recaro seats, and Apex ARC-8 with RS3's and Style 32's with Pilot SS for autocross