See the paperwork for all the KC element and for all the health tests.
All the things daisy said well.
The breeder should be will to give you lifetime advice, be willing to be responsible for the pup should you be unable to keep it, for whatever reason.
You might want to ask why they are breeding? Essentially, it should be because they wish to retain a puppy for themselves and be for temperament and breed conformity. Not so much this if working lines, differing reasons. Our breeder of the pup we pick up on Saturday works his dogs, so he wanted a pup from this bitch as she is excellent in the field, not sure he's so fussed about her looks 
You might ask about visiting the pups often, maybe photo updates and videos, not essential but nice anyway.
Little things can be nice too - our springer goes to his breeder if we go away without him, so he visits his mum and gran
Again, not essential, but it shows she cares.