The biggest concern I can see is the length of the RB motor into that shallow but wide engine bay of a 300ZX.
Next would be the mounting of the motor and gearbox. The GTR gearbox is a 4wd unit and will NOT fit under your zed properly. You are better of selling that (worth quite a bit to motorex) and picking up an RB25DET 5 speed. That gearbox should be fine to about 475- 500hp before you need to go to an O/S giken gear set.
A stand-alone computer system will work fine, providing your current instrament cluster runs off a factory stand a lone system. I would suggest a wire in system. Plug in is fine but unless you have someone with the complete wiring diagram in english of the skyline loom you will spend a fortune having it traced and such.
Go with an SX fuel pump for that part and get larger fuel lines feeding the fuel rail from the tank.
You will also need the front mount as the factory Zed I/C's are not even close to being effient enough for what you want.