I went with modded stockers. Mine are not the same as yours but they came out to be 810cc which was plenty and pretty cheap. The original owner was running stock injectors, don't ask.
You want 650cc or bigger for the 18G. Don't neglect the intercooler as well! Even with my regular 16G, I'm intercooler limited (and my STi intercooler is much larger than yours). Save some bucks for a nice big TMIC. Perrin and APS come to mind.
In another few years when the wife lets me spend money again I'll be looking at a APS CAI, APS TMIC, TD05-18G 7cm, and a re-tune.
04 STi 16G powered lagless fun
04 WRX: SOLD