Often just plain mutts make good ones. They can live off scraps and what they find, plus a little dogfood. The one I have now is a real jewel. She is friendly and playful but when needed can be mean. I watched her kill a large coon one night and I was very impressed. She has a little bull dog in her and she literally clamped down on the large coon's neck and held on for around 15 minutes straight.
I always know when someone comes up and she will not let a stranger in the house. She somehow senses relatives and is friendly to them.
As to home invaders, I don't make any elaborate plans as I don't expect anyone to get inside. Just in case, I do keep a loaded Browning HP beside the bed on my side. In the room I also keep an asagi spear mostly because I like it. I also keep a Winchester defender, usually loaded with oo buckshot, although sometimes I think a different load is better and for awhile I might use high brass BB shot, or even # 4's, I also see some merit in the smaller buckshot. I would prefer a Remington 870 but the Winchester was a great buy and is reliable. The shotgun is really more for outside emergencies as the pistol is much handier inside.
When all is said and done their is nothing better than a good dog for both warning and protection. Well, maybe nothing except a good gun.
I have never had to enter a house to hunt someone but have done some surveilance and have a couple of good "war stories" to tell, although by war stories I don't really mean "war" stories.
It's surprising where they can hide when they are very motivated.
A pair of dobies are hard to beat. Short haired dogs are a little easier to take care of.
When I was in K-9 we took our dogs home with us. Both of my dogs would sit and look out the door. They'd fire up when people would walk down the sidewalk in front of the house. Alot of the rats in the area would actually cross the street a few doors down from my house because they were afraid of the dog. And all the neighbors loved them.