What mxfly said.
If you can get a hold of these,
R34 GTR V-Spec II N1 (about 45 were made)
R34 GTR V-Spec II Nur
R34 GTR M-Spec Nur
R34 GTR Z-Tune
The Nismo Tunes were more so for ppl who have already got a GTR, take it into the Nismo Omori Factory and get the Nismo upgrades done.
The levels of upgrades are,
S-Tune Sports Resetting
S-Tune S1
R-Tune R1
F-Sport GT.