Good choice on the tire, Rbf600 for fluid and braided lines are a good first step. I wouldn't bother with the disc just yet. It is widely accepted in the S2k realm that stock discs are usually the most cost effective solution (don't know if this is the case with the 370z as it is a heavy vehicle). I've spent a lot on different discs and ended up going back to stock and replacing more frequently.
I wouldn't run a harness without at least a roll bar in the car. It can be done, but its just not safe without it. I've been to a few tracks where this actually a rule.
Finally if you do end up getting a harness make sure you know how to strap in or else:
ScienceDirect - Injury Extra : Scrotal injury following restrained impact: A risk of misnavigation