I found a cheap engine to swap into my Impreza RS and just wondering how much work it would take

You're putting the WRX engine in the RS?

 

If that's the case, the transmission bolting up is the least of your worries. Swapping a turbo engine into a non-turbo car is a big undertaking. Engine crossmember and sway bar have to be swapped. Radiator, exhaust. Most of the wiring (which requires removing the dash). If it's a manual transmission, you'll certainly need a new clutch. If it's an automatic, you'll have a nightmare of getting the electronic transmission control working.