I have a bit of experience with the EA82T
The first thing I can tell you is that it won't just bolt in to your impreza. Its a different family of motors.
YOu would be better off finding an EJ20G and stuffing it in there.
Either way you would need the wiring harness and ECU for whatever motor you decided to use. If you decided to go with the EA82T, you would need a trans adapter plate as well.
How much HP does the EJ18 produce. The EA82T only produces 111-115 stock