the rb motor is too long for the regular engine bay of an impreza, but if you modified the front crossmember for the radiator it could fit. Custom mounts will probably have to be fabbed up, nothing serious there.
AWD setup would be nice also.
ive done rb swaps in 240s but their engine bay is longer and I still had to put the fans on the front of the radiator. Anything could be one though.