ive just gone through this business and this plan works best.
start with exhaust it makes a mega difference for mine. 3" from dump, then cold air induction, then intercooler, boost controller.
these are pretty cheap mods, all up it cost me about $2000 for the above and thats all installed.
i g-teched my car to start with (auto trans) it was about 0-100 in 7.5 to 8.0 sec, now its about 6.5.
i bought my intercooler from ebay for around $300 new. the GTR coolers are good but they wanted $400 at a local jap import place and that was second hand. better to buy a new one. plus they look good too.
if you got the cash from there then id go chip,injectors, fuel system, turbo upgrade. the S15 turbo wont give you much of an upgrade from the attesa for the price i think they may spool up a little quicker.