Don’t think in terms of “puppy”. If you get a puppy it will grow into a “dog”, and in less time than you realize. So don’t just think of how (relatively) insignificant the weight, size and habits of a puppy are; consider the dog you’ll have in a few months.
There’s no way to generalize about “puppies” in the abstract like this. Puppies that you get from reputable breeders will generally be healthy, because the breeder’s reputation depends on that. (That’s how they get to be “reputable”.)
Puppies from “puppy mill” operations may be healthy (you could get lucky), but that’s just coincidental. The conditions under which those puppies are bred, maintained and rushed to pet stores “while they’re still so cute” makes it unlikely to expect “good health” from such a beginning.
On the other hand, the mutt you pick up at your animal rescue shelter may or may not be healthy (and if the place is run well they will attempt to steer you toward “healthy” pups unless you have a lot of experience and desire in nursing sick animals to health – and a proven track record of having done that), but at least you will know everything that they know about the animal before you take it. They have no profit motive in foisting an unhealthy animal on you “to make a sale”; their purpose is to match needy animals with loving and capable owners.
That’s the way I got Willow eight years ago, and we’re both glad I did.