Hello Mytran,
If you want a guard dog, unless you have lots of experience in training dogs, you should not get a puppy. There is a lot more to being a guard dog than just looking fierce and barking. If you don't train it properly it will be useless and could actually be dangerous.
Invest your money in either a young adult dog, that you will have professionally trained or in an adult dog that is already trained for the purpose.
Don't expect that having a dog is the "ultimate" protection for your house and belongings, it isn't always so.
Make sure that whatever dog you do eventually obtain gets trained NOT TO EAT ANYTHING THAT YOU OR SOMEONE IN YOUR FAMILY DOESN'T GIVE HIM/HER TO EAT. Especially not to eat anything that is on the ground. This is extremely important since many criminals who break into houses specialize in breaking into houses with dogs, because they genereally have more valuable belongings. They drive up, throw poisoned meat over the wall or fence and drive away. They come back later once the dog is dead and no longer poses a threat.
Actually it is not as difficult as you might think to train a dog not to eat, so don't let anyone tell you otherwise. It can be done, it isn't difficult and actually should be taught to ANY dog, not just guard dogs since it could save your pet's life.
If you are looking for a guard dog and you have children you need to decide whether the dog is going to roam free during the day, and therefore interact with your children and other members of the family it must be "family friendly", or if it will be kept in a kennel during the day and free only at night while your family remains indoors.
If you intend to have the dog roam freely during the day then you need a breed that is loyal and will be well comported with the family. It is for this reason the dog should be professionally trained. The risks posed by an untrained or unprofessionally trained dog are just to great.
Good breeds commonly used as guard dogs are German Shepherd, Doberman Pinscher, Rottweiler and Boxer because of their superior intelligence, size and strength.
No matter what kind of dog you have, or what purpose you have it for all dogs require lots of time, attention and exercise. If you don't have all three to give then DON'T GET A DOG, not ever. You will probably be better off by contracting a guard dog service than buying a dog that you haven't the time, skill or energy to care for on a full-time basis. Dogs require every bit as much attention as your children do, perhaps even more.
Cheers,
William James Woodward - Brazil & Canada Expert, Expat-blog Team