all breeds can be dangerous in the wrong hands and without the correct training. as for food, the slightly more expensive ones are better as they are not full of as much crap so you actually dont need to feed as much as you would the cheaper brands, so they may seem dearer to buy in the first place but are cheaper or equal in the long run.
if you are worried about your puppy with other dogs make sure you do plenty of early socialisation after its had its vaccinations. vets usually run puppy parties.
my DH used to breed bulldogs and mastiffs and we have a cocker spaniel and i have had lots of experience with various breeds in my work as a veterinary nurse, and i have known lots of families with staffies. its all down to training.