Welcome to NICO!
First off, lets get some things straight. You have a 2008 G37 coupe without NAV or a backup camera. Are you referring to the OEM screen?
Unfortunately, you need a new stereo unit to add NAV as it's hard drive based. I think you also need the touch screen monitor or they are sold together. For the back-up camera you need the hardware only I think. (the camera itself and the control module) I'm not sure if the stereo unit needs anything...that's something our techs might be able to answer.
Honestly, if you don't have NAV installed, it's cheaper to go aftermarket.
Here is a site that sells OEM parts cheaper than a dealer if you want to try and piece it together:
http://www.trademotion.com/partlocator/ ... atalogid=0