Which Wheels Should I Go For?

AntiVenom

9.5 all around if you are going squared setup (same size tires front and back) would be my pick.

assuming you go with the m510 I'd go 9.5 front 11 rear with 265/35 front 305/30 rear. this is a forum favorite for 19" wheels.

as for 40 offset on 9.5 all around I think you'd be starting a new trend called hellatucked where the goal is to not see the wheel at all. I jest but certainly you'd want spacers for a flush look especially in the back.

advantage of going with the 11" on the rear is you can get custom offset according to the getyourwheels site. so no spacers in the back. probably recommend a +15-20 offset.

also to be that guy: search the forum there are hundreds of threads on offsets, wheels, tires, etc. this should get you started though [Official] 370z wheel fitment guide (pics/specs) only & Wheel Spacers 101 (vicious's post in particular)