Your breeder should be willing and happy to let you contact any owners from previous litters for their experiences, be willing to give you lifetime advice and support. We also got tons and tons of emails and pictures while the puppies were growing and we could see them as often as we wished. Our breeder would prefer our puppy goes to her during holidays rather than kennels!
Yes to certificates and also, ensure you take any paperwork relating to KC registration with you, it should be ready when puppies are ready. Will the puppies be microchipped, docked (if appropriate), dew claws removed, first vaccinations etc? I think you can check the sire and dam on the KC website but not sure how to do so! Will you be provided with a puppy pack? Bit of food? Blanket from Mum? Perhaps ask how many litters from the same bitch? KC allow a max of 4 (I think), most breeders wouldn't do this many or breed from a bitch over 6 (again I think this is the age)
Some breeders do a puppy personality test so match you to the best pup and don't allow you to choose. You should have to go on a waiting list, a reputable breeder will have a list, not puppies available. We were lucky in this regard as our puppy was from a litter of 13, we were last on the list!
Does the breeder have children? Will the puppies be well socialised to people, noises etc? Good food from the outset?
Try to buy from a reputable website if you use this means, we got our puppy from Champdogs.co.uk as this website will only sell pedigree puppies (and checks)