Get a contract in writing before you agree to buy the puppy. The contract should specify what the breeder will do for you if there are health issues with the puppy and how long the guarantee is good.
Return of your money is best, but most breeders want to give you a puppy from a future litter. Also, most breeders will not pay for medical treatments your pup may need. They want you to fall in love with the puppy and then their offer to have you return the puppy for another in the future gets them off the hook as you will refuse to return the puppy.