This is what i did:
Do major service replacing all fluids engine, diffs, gearbox etc. Change timing belt, cam gears water pump.
I also changed the clutch because i knew i was gonna change the turbo's later.
Change the exhaust it if it doesn't have one. change the front pipe and cat at minimum.
If you still got money left over buy -7's or -9's.
Get a Nistune, power fc, Link or whatever your tuner prefers depending on your budget and get a tune.
At the end of the day you get keep the car stock but a stock GTR isn't that exciting.
DO the maintenance stuff first.