I'd say that almost any dog is OK, as long as he's trained properly. Small dogs would be a "no". My cousin (who has the qh's) has a Jack Russel names "Snappy" that truly lives up to his name. I'd say a "no" for him.
I'd actually suggest going out of the big busy town to a pound out in the country. Many of the dogs you find there came from people who either owned cattle, horses, or some other types of livestock. And also ask questions.
I don't like people going to breeders and stuff to get dogs that aren't for shows, just because there are so many dogs in pounds and spcas and humane societies all over the world, who are GREAT dogs, and just want some love. (most of the purebreds I know are VERY snooty patooty)
I would highly highly HIGHLY suggest you getting some mongrel mix. One that preferably has either some lab, or some hound in them. A bit of shepherd is OK, as long as it's balanced out by hound or retriever. Most of the hunting dogs tend to be OK around horses, seeing as most of them were bred to hunt alongside them. Just a bit of socialization, and learning that nipping the horses is bad, (sometimes it takes a soft kick from a horse) and you've got a great dog.