Are you committed to this puppy?
If not questions I would ask are:-
Have they health tested as a minimum for the following:- (ask to see paperwork do not just take their word for it.
Acral Mutilation Syndrome (AMS).
Familial Nepropathy (FN).
Progressive Retinal Atrophy (PRA-PRCD).
Degenerative Myelopathy (DM)
Exercise-Induced Collapse (EIC)
Glycogen Storage Disease VII (PFK Deficiency)
How are they socialising the puppies before they are homed
Are they familiar with ENS (early neurological stimulation) and ask to see there plan on how they are using it with the puppies. They should have a written plan that is documented for each puppy.
If they do not have any of the above I would walk away and carry on with your research.
They should be able to help you with the transition if you do go ahead.
Make sure they do not let all the puppies go to their new homes on one day to alleviate stress to the bitch
Make sure they give you the identical food they have been using for you to use at home to begin with
Leave a small blanket with them that can come home with the puppy that will have the smell of their original home on to help them settle
Ask the breeder to take the puppies out together in the car so their first journey is not on their own to the new home
Ask the breeder to give them very short periods alone without littermates so again it is easier when they are taken from the litter
Check that they will have their first vaccinations and which ones their vet uses - check this is compatible with the vaccinations your vet will use or else you will have to start again
Check they will be microchipped before coming to you
There is more but this is a good starting point