As the other anon said, guard dogs are mostly a deterrent. If someone was really after you or something you have, a few dogs won't do much to stop them. Keep in mind a dog can only stop a bullet once before becoming incapacitated.
That being said, you may want to think about getting any dog you get to go through schutzhund or something. That way when something does come up, you can give them commands to attack and they will be much more effective at taking a person down, and you can also give the command to let go which will be important. If you have a trained attack dog, many landlords won't accept your dog as well as some house insurance companies, or it may raise your insurance.
As far as breeds, just about every mollasar dog and live stock guardians are going to be naturally wary. Just Google for those and check out the breeds, find one that works for your lifestyle. Bully Kuttas, Georgian Shepherd, Kangals, akbash, antolian Shepherd, etc. Keep in mind most of these dogs will be pretty aloof so it is important to get a really solid bond with it. Being a part of the shutzhund training will help, as will basic training and occasional walks. Training goes on for the dog's lifetime, so continue practicing shutzhund even if your dog passed the class. You should probably put guard dog on duty shit around too to help in case your dog does bite an innocent. Never ever let children play with the dog unsupervised.