Hello and welcome! If you're looking for a mixed breed with a certain appearance, I'd suggest checking Petfinder for the breeds you like best, as well as your local breed specific rescues, regular rescues and shelters. I've seen many Corgi & Aussie looking mixed breeds that need homes, and German Shepherd mixed breeds are even more numerous, sadly. There are so many good dogs dying in shelters and living in limbo without permanent homes that my opinion is that folks should adopt whenever possible. If you're not planning on showing or titling your dog or doing other things that absolutely require having a purebred, saving a homeless dog would be a wonderful option.
If you do opt to get a purebred, please please please do lots of research and go to a reputable breeder who includes a statement in the contract that you can return the dog at any time for any reason. That way you'll know that you're supporting someone whose pups aren't going to ever end up adding to the number of homeless dogs. You'll also want to know what genetic tests should be done for your breed of choice and make sure the breeder that you pick checks all of that.
These two threads here are a good starting place to learn what else to look for in a reputable breeder. The last thing you want to do is support a back yard breeder or puppy mill for all sorts of reasons, but especially if you want to make sure you get the healthiest dog possible. https://www.dogforum.com/new-addition...ulation-88242/ https://www.dogforum.com/new-addition...reeders-88226/
However long a dog lives, it's never long enough, and even the healthiest, most well bred dog can have unforeseen health problems. That said, I'd think that feeding a high quality food, keeping your dog vaccinated and making sure he or she gets all of the necessary preventive care can go a long way towards keeping your dog healthy.
Good luck in your search and thanks for asking questions and being willing to do the research in advance.