The background of cavaliers is very important if you are buying one from a breeder. There's such a high incidence of severe and potentially costly medical issues -- in particular MVD -- that you want a breeder who cardiac tests breeding stock and health tests generally, and knows how to select the right pedigrees (all dogs bring some 'bad genes' into any potential mix and you don;t want to get two sets of the bad genes, nor do you want one of the parents to be known to suffer from any health issue).
The difference in life expectancy of a cavalier from breeder who breeds for heart health and one who doesn't can be a third the length of the dog's life, so this is a very significant element. Many dogs with early onset MVD die of heart failure by age 6 or 7.
That's why it is really important not to breed without understanding all the health and genetic issues, and why it is important to look for a breeder who breeds for health when seeking a puppy. Taking on a rescue dog or a dog needing rehoming is another issue, but I would never, ever go to a breeder who isn't knowledgable on health issues in the breed and working to keep lines as free as possible from ailments. In Ireland it is very easy to end up with a puppy farm cavalier of extremely poor quality so anyone buying in this market needs to be very vigilant.