You do want an experienced breeder.
You do want a breeder that health tests and uses a heath tested sire. I disgaree that it takes year to find the right sire. Dog people move in dog circle and they know the sire background from generations.
There is so much more to breeding than just health tests behaviourally wise all the major development stages are done in pregnancy and in the first 8 weeks of life . It is vital you have a breeder that knows what they ae doing in this critical time.
It is easier to get a pedigree as it is extremely to research the breeder from their breeding affix usually back several generations. Speak to breed clubs, speak and ask questions on breed facebook pages, research the breed and the same name will come up again and again. Speak to them and ask about their litters or litters that they recommend. Ask to speak to people who have had a litter from them previously, speak to the sires owner. Dog people love to talk about their dogs.
Cross breeds it an be done but is so much harder. Not all cross breeds but many crossbreed owners are not as clued up and do breed to make money rather then improving the breed. So issues will ocur.
Rescue dogs - the idea that a puppy is a clean slate and a rescue dog has issues is incorrect. At least with the rescue dog you can see before you buy and you know what the dog is like - with a puppy as I have said already the slate has been well and truly drawn on by the time you take your puppy home at 8 weeks and you will have no idea!
Howver rescues could have orginated from puppy farms and you will have little idea but then that is the same as buying off gumtree.
In short a good breeder will have a massive trail (easily found on the internet or from the breed club) that you can follow to get some indication of the way they breed, the dogs they produce and their general ethics to dog breeding. If you cant find a good trail walk away