I have some knowledge with breeding and raising rotties and my advice on this the following. Pit Bulls can be great dogs IF they are trained properly. Have you had a chance to see how the dog is around the current owner? These types of animals MUST MUST MUST be shown who is the alpha male and they must be shown that from the moment you take them home. I don't mean beat or hurt the animal, but showing you are in charge, ABSOLUTE charge is the most important thing you can do. This is a great link that explains how to show your dog you are the alpha male/pack leader.
http://www.canismajor.com/dog/alpha1.html#Stare
I cannot stress this role enough as I have had people that were timid in their dealings with a rottie of mine and had to be taught how to control this new animal they purchased. Pit Bulls while not that large are able to pull 30 times their own weight and are the most tenacious fighters once they are set off; so don't set them off.

By leading them they fall into a relaxed confidant role as your dog and will/should be fine.
I had one dog bite me a few years back and he bit me hard when he was about 1.5 years old. He had been testing me and my role as leader and it came to a head. He bit me and I did as I was instructed by the breeder that got me started. I grabbed the dog by the neck, literally drug him around to the pins where the other dogs were at and proceeded to beat that dog with stick and then my fists to within one inch of his life. I held that dog there and for about 2 minutes straight and hit it everywhere but the face. Nothing to inflict permenant damage but to inflict punishment and to enforce my role as alpha in front of everyone. That dog bites me, then my wife and any kids we might have and I will not allow it as a responsible dog owner and breeder. I want even keel animals to love and protect not to harm needlessly. After I finished punishing the dog I threw him back into his pin and looked at each dog in the eye. They all sat quiet and stared at me and I walked off. From that day forward my dogs were the best behaved animals I have ever ownd and a joy to be around. The one that was beat is my favorite animal and is always by my side; he is my best friend that does not fear me, but respects me and I him. I established my role without any doubt and you must do the same. I don't mean that to mean beat the animal, but gently but firmly instruct and lead.