If your not wanting to breed from this dog/bitch then the KC papers don't matter,
Once you get the puppy you will not be able to register it with the KC as the litter hasn't been registered by the breeder only they can do this,
so it seems one or both the parents of the puppies are not registered

if your happy with the pup then £300 is a fair price imo & 6wks is ok to take them home, i know a lot don't believe in this but at 6wks they should be on solids & not feeding off the mother