I would suggest getting a puppy from lines that are "protective"- not highly protective- once again, with good nerves. My dog was protective- but not highly so, or sharp, or aggressive. He made a ton of noise if a stranger came around- but he knew the sound of everyone's cars and if he knew them, it was fine. He never postured to bite, he just made a lot of racket- that's what you want, unless you are going to work your dog in bitesports. For example, my mom said he was a wuss- she came in my house, walked past his crate to the back of the house, and he barely woke up from his nap enough to look at her- but he knew her and the sound of her car. When the plumber came, he raised immortal cane and the plumber was scared to death of my 85 lb 9 month old puppy, lol. So as a woman alone, it was a good thing for him to know I had a "scary dog"- but if I'd introduced the plumber to him, he would've just slobbered him to death with kisses.
Training is key though- you have to be willing to take them to obedience classes. Nothing like a large dog with protective instincts that won't listen. My dog could've easily dragged or knocked us down- but he didn't try. I once spilled an entire bag of treats (including hotdogs) right at his feet in puppy class. He started to go for them, I told him to "leave it" and he stopped and looked at me. He didn't even eat his food until I told him it was okay to. With larger, smart, protective breeds, per se, you have to set a clear "leadership" role from the minute you bring em home! Firm but fair. And if your wife is home, she should be a big part of the training too
