As a person who buys LGD and does not breed them, my best advice is buy from a breeder who has being doing it a long time, and runs a commercial sheep operation. I add the commercial sheep aspect because i don't think a breeder can know what their lines are doing, unless they have enough work to test the dogs. I feel the same way about stockdog breeders.
A good breeder will walk you through the rough spots, and with many young dogs there will be rough spots.
I always tell people who ask me about LGD that if they have no tolerance for behavioral issues, and are not willing to work though them, don't get a LGD.
Right now several of my neghbors are loosing sheep, and chickens and cats to a fall blight of coyotes. They don't have LGD, but they do have llamas. We are not loosing animals and do have LGD. Evey year when we move our LGD and sheep from next to the neighbors sheep, down closer to our house to lamb, they loose lambs the next day.
I have had issues with most all my LGD. They did not come perfect, but i did what the breeder said, and we worked through it.
Dorchester, i wish you luck, it sounds like you have had a rough time, and don't blame you for your caution.