When I think of any Subaru from the late 90’s and 2000’s, I think head gaskets. See if your engine has been changed, which is most likely a good thing. Or that you prepare for the scenario that the head gasket may go. 

Kaizen is right about head gaskets, though there is a good chance that would have been replaced when it came out of the donor car.

Do you have any more details about what engine it is?  IIRC, there are a couple different EJ20 motors in the Impreza that year.  I might be wrong, but I think there was a 2.0L na (Impreza) and a turbo 2L (?EJ205 in the Impreza WRX).

With either engine, the biggest thing besides the head gaskets is oil consumption, but other than that they are pretty solid motors.  Many still on the road.  Very simple engines, too, and extremely easy to do your own maintenance.

If I had one, I would keep it on a tight oil change schedule - oil & filter every 3000 miles.  I’d check the oil weekly.  And I’d change the plugs every 25-30,000 miles.  And keep a clean air filter in it, too.

Then I’d drive it til it dies.

Head gaskets for sure unless you know the replacement motor has the improved head gaskets... A good head gasket kit will have all the other seals and gaskets. Also, while the donor engine is out, I'd get a good timing belt, water pump and component kit for peace of mind.