To have even close to decent spool characteristics I would say a TD05 16g or IHI VF39 or VF34.
All can be had for 500 or less used and out of the bunch I'd say the VF34 for a couple reasons. It has always been considered "small" for the 2.5L, and it will peak a bit under or at 300whp, so it's probably the smallest turbo out of the bunch (aka best spool if you don't want more power than that). If you want to go more then go for the 16g, the 39 is right in the middle.
Other items you WILL need:
At least 650cc injectors
Some sort of engine management (COBB Accessport or Opensource Tune)
I'd say the necessary stuff above will cost you a bit under two grand if you get a used turbo and mid-level other parts aka $500 intercooler, $300 dp. It will go up if you buy the pricier parts.
Things you DON'T need but are helpful/fun:
Cat Back Exhaust
Intake (must be considered in the tune)
Aftermarket headers or Port and Polished Exhaust Manifolds (I did the PnP route and love it- full spool on my VF52 at 3200 rpms)
3-Port EBCS to help control boost better than the stock 2-port
Add in all that and you're probably sitting around 3,800 or so depending on what brands you go with.
Then, of course, you're taking your transmission in your own hands. The early gen WRX's I believe have an uppermost limit at 300 or so wtq, but stock ones have been known to kill the transmission.