I used some RS*R springs in my Blitzen. I also had the shocks rebuilt at time. Nice ride, but still abit limited compared to STi. Might need guard rolling if wheels are larger.
Adjustables are hit and miss second hand. If you were to get some, then you might as well factor in a rebuild plus cert costs.
Either look out for cheap second hand exhausts from wreakers or look at Fortyone.co.nz for new or there yahoo auctions japan service.
Stay with OEM airbox, do a Cold air intake. You can replace the inlet pipe with a Zerosports one for abit of bling.
I wouldnt go Fmic. Cost for more lag. The standard Cooler is pretty good, but few things are replace the plastic inlet pipes with some samco ones and open up the bonnet scope (most just put a v8 scope on) Water sprayer can be done to.
I did do a V8 I/C install, but was PITA and required few custom mounts, Y-Pipe mods, and replace clutch Master cylinder with one from a V7+
Depending on you skills in things with cars, you can buy the cable and with the use of ECU flash/Romraider give http://torquedperformance.com/aboutus.sc an email. Costs a flat fee of $150US. Highly Regarded tuner.