Just getting any random dog or, worse, one which you visually find very appealing and attractive but which may be completely wrong for your household, lifestyle and ability is a recipe for disaster. It is absolutely essential to consider whether the breed (or cross) is likely to be right for you.
See the bazillion collies in rescue - cute fluffy puppies, 'they're clever, they practically train themselves', stereotype of typical common country family dog, etc. Then they start herding and nipping the kids, displaying neurotic destructive behaviours (soiling, chewing, barking, lunging at cars, separation anxiety, blah blah blah), recall goes out of the window, dog gets rehomed. The working roots of the dog and their resultant huge requirement for exercise and mental stimulation has been ignored to the detriment of everyone involved, not least the poor dog.
Crossbred dogs and mongrels can suffer from exactly the same genetic diseases as purebred dogs. Having an exact record of the dog's provenance actually helps responsible breeders to eliminate these diseases. Five generations of dogs with low hip scores and clear eye scores, proof of ability in agility/obedience/tracking/even temperament/trainability in whatever discipline you want your dog for those multiple generations = something a GOOD breeder will be able to demonstrate to you and result in a dog that will be perfect for your situation. There are no unknowns there.
If this is not important to you, and size/appearance/working ability is not a priority, just an all-round family pet, you get one from a rescue where their temperament will have been well assessed and again they will be matched to your situation. You do not buy from the back yard breeder who is filling these rescues and who cares only about the £££ lining their pocket, not the welfare of the dogs they are producing.