Hi and welcome to the site -- sounds like you have lots of experience with different animals!
And it is great that you are already thinking so seriously about how to find the right breed and breeder for you.
All purebred dogs have different genetic problems that they are more likely to have, because breeding purebreds narrows down the overall gene pool. Breeding for a quality you want -- be it appearance, temperament, or good health in one area -- automatically narrows the pool a bit further and after a time, may bring forth other breed problems. So breeding is both an art and a science and the very best starting point in getting any purebred is to carefully research breeders, find out how to check if they are following good breeding practice (we have quite a few threads on this here, and more information in the Library section), and be prepared to pay more from a reputable, health focused breeder. A reputable breeder will also be a friend and a source of wise advice and guidance throughout your dog's life.
There are some tools and protocols that do help reduce the chance of a dog ending up with these conditions at an early point or developing symptoms (sometimes you mainly want to avoid serious symptoms becuse the chances are high the dog will acquire the condition). For MVD, good breeders follow the MVD breeding protocol and cardiac test all their breeding dogs and select for long-lived lines with good hearts. With SM, things are more difficult -- most cavaliers start out with a skull that is too small for their brain and this in turn causes many, probably the majority, to develop SM over their lifespans. However very few of those are outwardly symptomatic and seem to get on with things despite these difficulties. As yet, very little is known about how best to try and breed away from it but more and more breeders are now MRIing their breeding stock and following the suggested MRI breeding protocol. This is based on the same general idea of the MVD protocol -- that you identify the more affected dogs and remove them from the breeding programme.
That said, your concerns probably will not go away by the time you are thinking of a puppy as these are very difficult conditions to limit and not well understood. But hoefully we will know a LOT more by then.
Right now, half of cavaliers will have MVD by age 5 or 6, and most will have it by 10. Increasing numbers of younger dogs are showing up with symptomatic SM. There are costs (in the case of SM, often very high costs) invoved with treating these conditions and they are very difficult to see in your own dog -- though again, in most the symptoms seem to remain mild.
Anyone deciding to opt for any breed that has more costly and serious genetic problems that could show up in a dog need to be able to make that cost commitment -- consider insuring the dog, for example. But remember -- most of us have dogs that don't have serious problems and any dog, purebred or mix, is capable of getting any serious and heartbreaking condition.
It's good that you are thinking about all these things now and you'll still have lots of time before you will be making a final decision to learn as much as you can. So many dogs, mixes and breeds, bring pleasure so I am sure you will find just the right dog for you, whether it is a cavalier or some other lovely choice!
