Well two things come into mind.
1. That 5.0 will not reach your HP goals and still be DDable. Those engines are not that great. Your looking 300HP tops with that he has in there right now. To get it to your goals is going to require all forged internals and alot of weight loss in the car. Plus FI.
2. Buying a car with that much mods to it is just asking for trouble. You dont know if this guy knew what he was doing, if corners were cut, how things are ran. You could spend a week just trying to trace down a electrical problem. Just something to think about.