I think a border collie would do the job well. An australian shepherd
might do the same thing. I have a yellow lab...he's not what you are
looking for, as he gets along with everybody.
When Dax was alive, he and Benny would make a great, noisy production
when anyone approached. Very impressive. At least two people told me
they were more nervous about 55-pound Benny than 110-pound Dax because
Dax would stay in front of them and Benny would get behind them.
But people told me that when I was not here, both Dax, the German
Shepherd, and Bennie, the Aussie, would not even bark at them. Make of
that what you will.
A nasty junkyard dog might well be intimidating. It might also get you
a big lawsuit. If gentle people were approaching the house a nasty dog
might hurt them. If a serious trespasser was approaching the house he
would simply shoot the dog.
My point is to carefully consider what it is you wish a dog to do.
Seems to me about all any dog can do regarding security is to warn you
that someone is coming.
Guinea fowl do that very well and they eat ticks and have lower vet
bills.