The best advice I can give anyone buying a puppy is to remember to obtain copies of health clearances for both parents. The more health clearances the parents have, the better chance you have of purchasing a healthy puppy. The cost might be more up front for the pup, but I believe you would get that back on what you will save on vet visits.
Each breed of dog is prone to breed specific defects. I will provide a link in this article so you can research the breed you are interested in. I believe the three most important clearances to look for are hips, elbows and eyes. Hip and elbow information can be found at the OFA (Orthopedic Foundation For Animals). It is important to obtain copies of the certificates for both parents of the pup you are interested in. Eyes should have an annual evaluation by a veterinarian ophthalmologist and be Cerf certified. Some breeds are prone to Prcd-pra which usually causes blindness. The pups will not inherit the disease if one parent is clear of the gene.
If buying from a breeder, you should get a 72 hour money back guarantee to take your pup back after being checked by a vet and found to be sick. Also, your contract should have some guarantees with it such as hip dysplasia, etc. Your contract should state you are entitled to a full refund should your pup show signs of hips, eyes, etc within a certain period of time….minimum of 18 months.
Chances are you will not get those clearances when buying a pup from a pet shop. You will know whether or not you are buying from a reputable breeder when you start asking questions.