Shelter dogs may have problems. With breed specific legislation in most states you need make sure you have a stable animal after you chech local laws and make sure the Pits haven't been banned in you area.
I would definatey recommend getting a puppy. I have had many Pits and the bond that is developed at early ages is important.
Right now I have a Male ,2.5 yoa that is absolutely one of the most loving and intellegent pets you could ask for. But like any other high energy dog you have to exercise Pits. At least if you don't want stuff getting chewed up...lol.
I can ask him if he would rather drive a Ford Mustang or be dead, and he'll fall over "dead".!
Check out the kennel and ask some questions. There is a difference between "pet bulls" and game dogs. Getting an animal from a shady kennel with the breeding dogs tied to short leads that looks like the dogs aren't loved at all could come back to haunt you. If the owners are scared to let you touch the parents you might want to keep looking.
There are pit bull forums...do a little research there.