There are two styles of H&R Spacers, the DRM (bolt-on) and DRS (stud replacement).
The 20mm or larger DRM bolt-on style will not clear the factory front wheels, but the DRS stud replacement ones will.
We're about 10 to 1 on people going with a 15mm front and 20mm rear, but we've definitely had guys mix it up and be happy. It's a little bit about personal taste at that point.
We've had a customer nice enough to send us his DIY on the stud replacements, it's the perfect saturday afternoon project if you have basic tools.
Front:
H&R TRAK+ DRS 15mm Front Wheel Spacer Install – Nissan 370Z : STILLEN
Rear:
H&R TRAK+ DRS 20mm Rear Wheel Spacer Install ? Nissan 370Z : STILLEN
DRS are less expensive than the DRM. I'll PM you the pricing.