When I bought Max he had an underbite, I took Missy to a breeders house who had 7 puppies to choose from. Missy picked out the cutest one in the bunch, the breeder told me "You dont want him, he has an underbite" but he was so darn cute and it was love at 1st sight between him and Missy. I checked his underbite and it was very minor, his bottom row of teeth was just gently in front of the top row his bite seemed pretty good other than the front teeth. The breeder was selling him for 1/4 less than what she was asking for his littermates so we took him
his underbite corrected itself once his baby teeth fell out and the adult teeth came in, he now has a perfect bite.
I would not consider buying one with an underbite unless the price was lower than 1,000 and only if the underbite was very slight and the bite looked good over all considering.
Good luck with your search for a puppy, take your time and I am sure you will find just the right one