I'm a little confused as to whether you want a herding dog or a flock guardian? They are two different kinds of dogs, and I can't think of any breed that does both jobs.
A flock guardian lives with your sheep full time, 24/7. Their job is to protect them and fight or scare off threats life coyotes. They bond with the flock so that the sheep accept them as one of their own.
A herding dog herds the sheep. They work with you to move the sheep from one location to another. The sheep should NOT be so accepting of a herding dog because the dog acts as an aversive that gets them to move. The dog should not be left unsupervised with the sheep, because many herding dogs will constantly work the flock, which is stressful to the sheep. It could also lead to aggression towards the sheep as herding is actually fine tuned predatory behavior, if left unsupervised, its not so good.
A flock guardian requires little in the way of human interaction. They live with the sheep, most are indifferent to people. You would want to start with a puppy, so it grows up thinking it is one of the flock.
A herding dog requires a LOT of human interaction. While herding is instinctual, it requires a lot of training for the dog to learn how to channel the instinct, and for the human to learn how to coordinate with the dog. I've heard its actually quite hard. You can actually get lessons in herding, and it takes quite a bit of work, practice and training for both human and dog to get a good, functional, working relationship.
Guard breeds include- Great Pyrenees, Anatolian Shepherd, Kuvasz, Komondor, Marrema Sheepdogs, Akbash and Kangals
Herding breeds include- Border Collies, Australian shepherds, German Shepherds, Corgis, Shelties, Collies, Malinois, Australian Cattle Dogs, and Kelpies
The first order of business is deciding what you need. If you live on a small property, you probably don't need a herding breed. And since they would be in the house with you or kenneled away from the sheep at night, they probably wouldn't be much good against coyotes. I'd be inclined to think you would be looking more for a Guardian. its also worth mentioning, if you want a companion dog, their are other animals that make good flock guardians- Llamas and Donkeys can both deter coyotes. If you do want to herd, I'd HIGHLY recommend finding a shepherd in your area who uses dogs. They can help you assess your needs breed wise and help to train you and the dog.