I have to agree with those who have stated that you just don't buy a dog from a pet store. They are ALL puppy mill animals, to a greater or lesser degree.
If you really want a pure bred you have to do the research to find a reputable breeder. These people will insist on interviewing you to decide if you
are good enough to purchase one of their dogs. They will have the health records of all of their dogs, so you can view the medical family tree of the individual that is up for your adoption.
As stated before...all animals, especially elderly ones, are prone to cancer.
It is a very big decision, but it is a 12 to 16 year commitment (on average) so it is important to take your time.
Everything you described in your OP is setting off alarm bells for me. Proceed with caution. Good luck.