That looks a lot like my old 2002 WRX wagon! I had fun with mine, but they have their share of issues and can be finicky. Here's what to look out for as far as issues and maintenance options:
-At 100k, it's going to rattle and clunk all over the place. The rear diff bushings were made of peanut butter from the factory, so those will be first to go. As previously said, Turn In Concepts (if they are still around) makes a "Clunk Killer" kit with all the bushings you'll need to make that stop. If it had a M/T, I'd replace all the shifter bushings and get a Kartboy shifter, too.
-An airflow sensor CEL may be evidence that someone had a CAI on the car. With that said, look for evidence of previous mods. These don't like the typical "My 1st Import" mods that lots of people do to their Hondas. Check vacuum lines for evidence of splicing in a boost gauge, bad wiring, stuff like that.
-The early WRX's had a pre-cat in the uppipe that had a tendency to break apart, much like other cars of that era. Look out for that. Replacing it with a non-catted pipe is common, but you'll need a reflash or "MIL Eliminator resistor" to tell the computer not to reference the EGR probe that's in the pipe in the same way. That is a PITA. AND if you do get one, don't get one of the cheap eBay ones with the flex joint! I have seen the joint collapse in and fall apart and do the same as the precat would have!
-These cars are finicky with fluids. Mine didn't like Mobil 1 oil; it would burn it off. I used Shell Rotella 5W40 synthetic diesel oil and that problem stopped. YMMV.
-The stock radiator is probably on it's way out by now. There are lots of companies out there that sell affordable aluminum replacements.
-There are a million and one options out there when it comes time to service the brakes and suspension. That's a good thing. I ran Hawk HPS pads on my car with parts store blank rotors with great success. (Yes, you just read that in Borat's voice.) My wagon was running the factory JDM STI "pink" springs with Tokico adjustable struts, and it was magical.
-A 20mm rear sway bar from a 2002 WRX sedan and an eBay rear strut tower brace stiffened up the rear enough to make the car "just right" for fun driving.
-If he lives were it gets really cold, prepare for fuel line leakage. They have a tendency to shoot raw fuel all over the engine under the intake in temps of 15 degrees or below. There was a recall out there, but only for certain states. When it happened to mine, they didn't consider Massachusetts to be a "cold weather state". It was $650 for the dealer to do, or about $10 in new clamps and 6 hours of my time.
-At 100k, it's due for a timing belt, and all the other fun stuff that goes with it.
-It will get abysmal gas mileage. Mine had a manual, and the best I could get out of it was around 23mpg. The auto will likely get worse than that. And they drink premium.
As others have said, he might be better off looking for an Outback Sport with fresh head gaskets and a timing belt and save a few bucks.