I'm not advocating either way, for or against rescue animals or breeders.
I will say that the "purebred" label generally is worthless where health is concerned. Hip dysplasia, for example, runs rampant in the GSD breed, especially in showlines. Generally, the best a breeder can do is give you a guarantee, that basically says, if your animal gets sick, you can give it back, and they'll give you a new one.
Hip dysplasia isn't even detectable under a year of age. It's unlikely anyone is going to have a puppy until 1-2 years old, and then give it back to the breeder in exchange for a new puppy. This is what the breeders bank on.
It takes a loooooot of research into different lines, breeding, pairings, etc, to have any idea of what you're getting in a pup, and even then, puppies are always a crapshoot.
That's why people who actively compete in Schutzhund will pay $1,500 for a good workingline puppy [mine was slightly less, owing mostly to the white spot on her chest, which is considered undesirable, although not quite faulty], but upwards of $8,000 for a "green started" dog [a dog around 18 months of age, temperament tested, sometimes alreadywith a BH title, and OFA Prelim Hips/Elbows.]