Just got a new puppy, but now we are iffy about the people that sold it?

We just got a 6 week old purebred black lab puppy yesterday, and he is as sweet as can be! I found him in the paper, and we paid 250 dollars for him. He has all of his boosters (which the people did themselves) and he has been de wormed. The people were very nice, and said they have been breeding puppies for 15 years, and that they used to charge alot more for them, but that they wanted the puppies to go to good families, so they lowered the price. Anyways after getting him home, we noticed he has fleas, also the people said the reason that they could go at 6 weeks was because they eat cat food. And thats its way better for them because it has alot more nutrients than dog food. So they said for us to buy a bag too, and about half way through it start mixing it with puppy food that way we don't switch right over. Anyways he is a very good looking dog, im just iffy about everything now. Were not gonna return him I was just wondering if anyone had an opinion.