Any dog can protect its family.. The best guard dog I've ever known was a medium-sized mixed breed from a shelter. She never had any protection training, but she was the perfect guardian. She never started fights, only finished them, and she was "just" a little mixed breed from the pound.
If you do want a puppy from a breeder, though, I would second Showpug's suggestion about looking into the wokring group first for potential breeds.. Although, I wouldn't recommend many herding breeds - Often times, herders can be "too much dog" for even good dog owners.