Yea, very few "common" issues on the '00-'04 cars (by far my favorite Subaru). 4-cylinder cars are notorious for Head Gasket leaks, but unlike the stereotypical head gasket failure, these almost always manifest as an oil leak. Annoying, but as long as you check the oil regularly and don't let it run out, they'll run a long time (I had an '03 that was leaking when I bought it, and I fixed it right before selling it about 40k miles later).
6-cylinder cars have an issue with the serpentine belt pullies that can fail with little warning. Replace them preventatively.
Depending on trim, Subaru started phasing in immobilizers into the '05-'09 cars. If that's the case, keep in mind a duplicate key is $150-200, and if you loose the last one, it's a HUGE project (make sure you always have a spare). Turbos were an option in this vintage, these are much more finicky about maintenance, I don't recommend them for daily use. Wiring into the tailgate is fairly common to fatigue were it bends between the gate/body. If it has any issues with the rear wiper, lock, or lights in the tailgate, there's a decent chance it needs repair there. Individual breaks can be traced and repaired, but it's about $80 for a new harness to reset the clock.
But far more important is general maintenance. Fluids, matching tires, stuff like that. I normally look for low-mileage, neglected cars for cheap. Last year I bought a '00 with about 320k miles on it from an acquaintance who I know takes good care of his cars. It has been eye-opening how much fewer problems it has...