Also, you need to decide what you want to achieve with your car. What kind of power are you aiming for? Example, you do what these guys have suggested (which I agree upon) but if you get bitten by the wasps, you are going to spend much more money replacing these parts you just spent money on. Highflow turbo will only go so far, same with you AFM, same with a remapped ECU, then your clutch, then uh oh you have a race car!
Once you start modifying, it doesn't stop so I hope your budget stretches
My suggestion, keep it simple! Your current setup should see 180 to 200 kws once retuned on the right boost and then keep it at that. My old 33 S2 made 210 with only a boost controller to 11psi, fmic, splitfire coilpacks (cause the standard ones died), exhaust and a tune from Toshi in this club.
You drive a Skyline, you can pick up chicks without having an 11ty thousand HP machine. Save your money, upgrade to a GTR. Chicks will then throw their panties at you.
edited out the coilpack remarks because I missed that you mentioned you had after markets