congrats! what i'd ask is about patella luxation. this is a lose kniecap which is a fault in hereditory! look if the pups have large enough nostrils as they can otherwise develop breathing problems. parents may seem healthy but ask to see vets documents. here you should be able to see whether the adults have been vacinated and if they have gone under any operations. ask to see ithe mother alone. then you can see if she is friendly and has no anxieties. females will give axieties to pups! pick up both pups one at a time. let them stand on their hind legs on your hand. if they don't stand evenly on your hand this could be the knie thing!
what i'd also do is come unanounced. then you can really get a feeling of how the breeder tends to his pups. this is no guaranty ofcourse but will give a good impression of a good breeder. the last thing i'd ask is what if the pup has breeding faults? will he help with the finances or will he give a new puppy when he's so ill that there not a thing you can do anymore!
ofcourse this is all precaution as he is probably an excellent breeder!

good luck!