Read
"The Dog Listener" before you buy a dog. If you follow the author's advice, you will be able to raise a puppy. But remember, a puppy will not be ready to protect its home until its about 6 mo. old. On the other hand, a "rescue" dog will also not bond right away. It may also not be a natural watchdog, so be sure you request that trait before adopting.
Another thing you can try is to foster a dog. I've done this before and had great success. You get the first dibs on adopting, and you get to work on your training skills with the dog.