I can tell you that used Subarus are like used BMWs, Mercedes and Audis. When they are running well, they are great, but when they need repair, you will pay BMW, Mercedes and Audi money. The WRX is a great platform, and the engine is its heart and soul. If he cannot handle the complete rebuild himself, he needs to look into a loan. It is the same with a 69 Dart with a 340. You either can or cannot handle the rebuild yourself. If you cannot, you have to hire it done. Fortunately a 1969 340 with a carb is FAR less complex and will cost far less to get running well. The WRX engine is complicated. Even if the rebuild goes well, your son's friend still has to deal with the fuel injection, emissions, computer stuff, etc. I wish I could give you better news, but there isn't any. He will likely have to pay big money to get it fixed.
This is why he got the car cheap. If it was abandoned by the owner, that should tell you something. AND, if the mechanic sold it cheap, that should also tell you something. If he could have gotten it running well, it would have been worth a lot more. The mechanic knew that could not have been done at a cost that would have left a profit for him (or her), so it was sold cheap. It's kind of looking at a 69 Dart with a 340 and a 4 speed. The guy says, "all it needs is a carb, and it will run great. But you can't hear it run, so you can't drive it, so for all you know the engine, transmission, suspension etc. could all be junk. The seller KNOWS he will get way more money for a running car. If it really DID only need a carb. he would find a carb to get it running. So the fact that he doesn't get it running should tell you something.