I looked into doing this.
Conclusions I reached...
-Don't get a used motor, you have little to no idea of the history
-Building a motor is rudely expensive, generally speaking
-Buy a closed deck block and build it on the side, that way you can do it bit by bit because you haven't killed your car... yet
Although the final conclusion was that after all that, I still would have to do a hell of a lot more conversions and when all was said and done all I would have is basically an STi that isn't worth as much as an STi because it technically isn't one, with a built motor.
So instead I sold the car and bought a V7 STi. So much stuff already done from the factory - forged bottom end, HUGE brakes, blah blah blah. Plus you get goodies like VVT. And realistically, for less money than I would have spent converting my V3 WRX.