Well I'll weigh in... I have an Australian shepherd, a large-breed herding dog.
First off, he is the smartest animal I have every been around. As far as brains go, the breed is right up there with the Boarder Collies, etc. Trainability is top-notch. Mine understood "sit" and "lay down" by 10 weeks.
Now I cant vouch for the breed as a whole, but my Aussie is VERY well-mannered. Never gets in the trash. Never jumps on counters. Never eats people food unless its handed to him. Wont get in his own dog food unless its put in his bowl, etc.
He is very protective of "his" people, home, and "siblings" (another dog and a cat). He is not dangerous to others but he definately meets strangers at the door with a great BIG bark. I can take him away from the house and "out of his element," so to speak, to the boardwalk or fair or horse shows or dogpark and he does just fine - no agression toward people or other dogs, horses, cats, etc. But I honestly feel if someone were to attack me, he'd be on their azz.
He is very gentle with smaller dogs, puppies, cats, kids, horses, etc. The breed typically dosnt play really rough so your little guys should be safe
I also think an Aussie would be good for you for fending off the buzzards because are ARE big (up to 65 lbs) and will investigate anything "forgein" that comes into their yard. But at the same time, a 65lb dog isnt
too big.
Keep in mind that they are herding animals - if something runs, they're most likely going to chase it... But like i said before they are very well behaved and easily trained so if you teach and enforce the "HALT" and "come" commands, you're good to go!
There are two types of Aussie breeders. There are "working" breeders and "show" breeders. The dogs that come from a working breeder have the herding instincts, the smarts, etc., but may not have a "show quality" coat. The dogs that come from the show breeders dont have such intense herding instincts, may be a little more silly and un-focused, etc., but have the looks. For what you want, I'd go with a working breeder's puppy.
If you want more info and/or have any more questions, please feel free to ask. I absolutely love my Aussie!