Buy OEM nav system and install it into the basic 370z?

Diversion

I never said I knew anything for sure. Unless you're referring to me saying it's not worth it.. because it's not. No matter how you cut it up, it's not going to be a fun job. And i'd hate to see the enormous list of things that you need to buy/change out to make it work on top of that.

Usually a car maker provides completely different dash part numbers to coincide with a Nav system. Which includes the differences of center console, wire harnesses, etc.

Just go aftermarket and save yourself from a nightmare. The best thing you could do is wait for a wrecked 370Z with the Navi system and pull the entire dash + ecu + a pillars, antennas/etc. In my opinion there's a lot more integration with the system than you think.