Hey,
I've done a few motor swaps on subarus. One was a 2.5 DOHC into an impreza that was a 2.2 SOHC. There was virtually no issues, in fact Im still using the 2.2 ECU and all is happy. I had to do a little vacuum line sorting out to stop EVAP CEL's. Mechanically it's just a swap, the wiring is where you must be careful. In the worst case you would have to pull the dash to change the wiring harness to the ECU, not a simple or short task.
Depending on the year differences you may have to swap ECU's, you might have trans wiring issues (disregard that if they are both standard). Carefully examine every single wiring connector on the donor and the recipient. That will give you a clue to the level of pain your going to endure.
All that said, some folks like the SOHC better (less HG issues), I went with the 2.5 DOHC because I knew the motor, the HG's & t belt were just done, I had a nice Borla header to fit it, and the 2.5 has ~25 more ponies than the 2.2..
Full disclosure, I've owned Subarus for many years, do almost all the work on my cars, (currently 4 drivers and 5 Subarus in the house), I find Subaru's are very easy to work on. Lego cars...
Given you have a complete parts car, if you are mechanically inclined and take your time, I don't think you'll have a problem. Now putting a JDM turbo motor from a pallet into a legacy was more.... challenging....
-- tip, do the t belt, idlers, plugs, PCV while the motor is on the bench... it's sooooo much easier.. valve cover gaskets too if required!
good luck.
