We've seen it. We know you need an engine. We know. Like I said spend a little on the engine and more on other things. Especially the transmission. Our trannies are the weakest link for anything over 300 horse. Most likely if you have 120k on it 2nd gear is about to go. Also you'll want a limited slip in the back.
We know what we are talking about and everyone here is going to tell you to get a fresh engine, mostly stock and spend the rest of your cash on the rest of the car. You can have a huge engine, but without the brakes, suspension, driveline, and a 20 year old car that is cleaned up, it's all worthless.
Oh and since your a noob I'm going to let you in on a secret, once you start a build it's not the big things that cost money, it's all the little things that add up. Fluids, bolts, sub-let work, machine work, hoses, belts, ball joints, etc. those add up fast, trust me. You can spend a hundred bucks on a handful of proper bolts for a mitsu.