Contact a breeder, and make sure they offer a health certificate that includes hips. GS are prone to dysplasia, which is a crippling disease. Spend some time with the puppies, and pick one that is aggressive, and attentive. You won't want the shy little pup that stays at the back of the pack!
Will the dog have a good run outside? My boys are outside in their run all day, then come inside to be with the family and sleep inside in kennels at night. If it's too cold, they stay inside all day with me running home at lunch for their pee break. Since you want this dog for protection, the more time your parents can spend with it, the better--you want that dog to consider your parents packmates. More time together equals better bonding, and more protective feelings.
I'd also encourage both parents to go to training with the dog. This will be more important for your mom, since she's the one the dog will be protecting, but having your dad there too will be better.
I'd go with a female. IME, females are much more protective than males. My males prtoect THINGS okay, but aren't as worried about people. All the female dogs I've ever owned have been very protective of people, not as worried about things.
Food: a good quality pet food. Research first, as 'expensive' doesn't always mean quality. Ask you vet what they recommend, and see if there's a large breed puppy formula available.
Maintenance: annual trips to the vet. If you aren't familiar with any vets in the area, schedule a visit before getting the dog. Talk with them, and pick one you like. Having a good vet that you're comfortable with is critical! Also, daily exercise! Running around in the yard is one thing, but getting a good walk in is really important, too.