1990 Nissan 300ZX Bent Valve?

I am considering a purchase of a very nice looking 1990 300zx that supposedly needs a new valve because it was bent after replacing a fuel injector and fuel leaked in. The person selling the car states that after he replaced the injector, it cranked once and the stoped. He says he can manually turn the engine 350 degrees before it stops. What do you think is wrong with the car, and how much should it cost to repair? Thank you so much!