As long as you have the parts ready, it can go very smooth. A day project for sure.
I would recomend buying a programable ECU and forget the stock ecu and harness. makes life easier.
Make sure you get the crossmember or have one cut and welded to strengthen it back up.
You can use the stock fuel pump. Its good for 75 psi and plenty of Gpm, unless you run 400hp
I would buy different injectors.
Clutch is a must, get a four puck racing clutch.
Pay someone to put it in and you off.
Haltech ECU $1300
Engine $1000-$4000 $1K for the old WRX engines
Crossmember $200
Injectors $350
Labor $0 if you do it.
Clutch $200 or less