Above and beyond an obligatory expression of gratitude: I am sincerely glad you are back from your military service, intact (albeit enduring a great deal of hardships to manage, the likes of which I can only barely begin to imagine), and also glad that you survived the crash -- you, sir, are tough as nails, a fighter and a survivor, and for that alone, I salute you.
It's a shame the GT-R is a wash -- but you have the picked up sweetest of Z34's. Continue to endure, survive, and thrive and, of course, do so in style, having fun with the new Z
So, with that in mind, my humble suggestions for what to do with the new car:
1. Intakes - easy and massive-bang-for-the-buck --> K&N (or similar) panel filters + cobb (or similar) smooth (i.e., not "accordion" style) intake tubing. Money to burn and bumper removal a fun occasional Sunday in the garage, Stillen G3's, for a little more top end and growl.
2. Exhaust. Keep the OEM H pipe and cat back one, but replace the OEM cats with either high flow (RESONATED!) variants, or, if for various reasons you would just as soon as dispose of the bottleneck entirely: resonated test pipes or long tube headers (recommended: PPE).
3. Suspension/Brakes: The Nismo is stiffer than the non-nismo; add additional bracing, if you wish, in keeping with your goals, and some consideration of how over/under/neutral you like to be. OEM is fine.
Likewise with alignment, but going lower will demand more camber correction. For dampers and springs, I say, forgo coilovers, and get a set of Koni yellows and Swift sport springs. There are detailed threads on this combo, but in short: lower, flatter, firmer (the OEM Nismos, I believe, are not progressively wound, and thus can be uncomfortable when not hunkering down to take a corner -- it's always hunkering down to the same extent...).
In theory, you have stiffer lines and pads with better bite already -- I recommend Goodrich SS lines and Project Mu pads if you want a bit more out of them.
4. Tuning: If available for your car, look into Osiris Uprev -- more of a necessity if you do the Stillen G3's and definitely if you do headers or test pipes. Otherwise, a bit less critical for panel filters+tubes+HFC's.
5. Tires/wheels: OEM is quite good on the Nismo -- but you may want to look into spacers -- say 20 mm in F and 25 MM in back. This is, frankly, more cosmetic than anything -- the effect on overall handling is nominal at best.
6. Fluids: Redline 5w30 seems to fare very well on UOA's, while also quieting our noisy VVEL heads. Eventually, you'll want Redline for your other fluids too. Pretty well liked around here, and so it seems, deservedly so.
7. Oil cooling: At need. See where you tend to hover. If you rarely creep past 230*F, try some Arc cool fins (search). Inexpensive but effective. If you will be tracking or find yourself running well over 230* F quite a bit, look into replacing the OEM cooler with an aftermarket oil cooler -- numerous threads on it.
Enjoy!
