Skip the VF series nonsense. They're not very efficient turbos and not rebuildable. They're also somewhat overpriced. They're popular because you can find used ones off of STi's, not because they're good. Look at a TD05 based mitsu turbo like a 16G or 18G.
For any turbo upgrade on your car, you will need:
-intercooler (top mounted is fine for a medium sized turbo)
and people will recommend:
-3 port BCS
-budget for a transmission replacement
2015 STi: Still Stock
04 STi, 04 WRX: SOLD