Oh jesus christ..... You made a foolish mistake buying a car without getting it checked out. Fine. Live and learn. But wait.... You have the option to return this POS car and cut your losses, but INSTEAD you have decided to spend FIVE THOUSAND DOLLARS on a new engine that should not have been blown in the first place, not including the suspension and other trashed up bits. Oh, also since you don't know jack about cars, don't forget to tack on a couple thousand dollars for labor. Or more.
This dealership that only sells cars that "matter" is taking advantage of you. They are feeding you bullcrap and right now you need someone to help you see straight. Your not thinking with your head. Your an easy target with $15k burning a hole in your pocket. You are being manipulated. You fell in love after one test drive of one STi. Purchasing a car should be as emotionless as possible precisely so something like this does not occur.
Take the car back. Keep saving your money and in your free time hang out on here and other boards learning everything you can. Never ever ever ever ever ever ever ever buy a sports car without getting it fully inspected. These cars are routinely beaten to within an inch of their life. Mark my words if you keep this car it will be a money pit and you will regret it. Learn then buy. Not the other way around.