As with any breed or when choosing a breeder, try to go around and visit a number of different ones to really get a feel for the breed. If you do find a breeder you like that is close enough to visit, do so and check out their dogs in person. Most good breeders will also be more than happy to give you the contact information for all their previous puppy buyers, so you can talk to them, perhaps meet their dogs as well.
If "all" you are wanting out of an Aussie is a wonderful companion and hiking buddy, I highly, HIGHLY recommend you look at your local chapter of ARPH (Aussie Rescue)
first, or at least at some point in your searching. This is where I got Sawyer from and I couldn't have built a better dog if I tried. You can find many puppies in there, if that's what you're after, as many people get sucked in by the cute fluff and forget that they've got a herding dog on their hands. However, if this is your first Aussie, I'll go ahead and recommend that you look at a dog that's a year or so old.
But if the breeder route is really the way you want to go, go visit their dogs if you can, or see if they are going to be at any dog shows near you. My big thing with show dogs is that not only do they lack the herding drive that the breed is intended to have, but that they also lack that all-important "off-switch". I used to work at a busy boarding resort and the pet-bred/showline Aussies that we got in were just...kind of dumb. They were like docked Golden retrievers...loved everyone, constantly bouncing and barking...this is a comical breed, don't get me wrong, it's one of the reasons I love them so much. But they should have a slight reserve to them in public, taking in everything they see, instead of blinding galloping around going 'HIIIIIIIII!!!!!!"
Sawyer is a very friendly dog in most cases, but his was of showing it is to stand next to a person and leeeeean into them. He goes to work with my BF quite often, who runs a showroom for pool tables and arcade equipment, so Sawyer meets strangers on a daily basis. Everyone loves him and thinks he's just the sweetest dog ever (minus the barking when he's playing with the other staff dogs, lol) but they don't see the pogo-stick bouncing he saves for greeting me.
I would look for a breeder that has a mix of working/show, or at least trials their dogs occasionally. For that, look more for ASCA breeders over AKC, or at least a dual-registered breeder. That way you're still going to get the bigger coat and flash you seem to want (though why is beyond me, Sawyer has a pretty moderate coat and he sheds enough as is, lol!) but you should still get a dog that has the brains this breed is meant to have. Otherwise, you've still got a very energetic dog, but with little going on up top to help control it.
The main diseases to look out for in any Aussie or breeding program are hip dysplasia, Collie Eye Anomaly and epilepsy. Hips and elbows should be certified by either OFA or PennHip, they should have a CEA clearance yearly and the breeder should have records or at least be willing to talk about any incidences of epilepsy in their dogs. Not all epilepsy is inherited, some of it is idiopathic (unknown cause) but it is a heartbreaking problem and one that most breeders are working to eliminate. Well, most of the good ones. There are some shady breeders that cull the dogs and hide the evidence of the disease in their lines, mostly limited to BYB's or show line glory hound breeders.
For this reason as well, assume your dog carries the MDR1 gene until tested otherwise and dont' give them any medication with ivermectin in it. Stick with Interceptor for your heartworm prevention. It's safer in general and covers more than any of the ivermectin based ones do.
Ok, that's all I can think of for this cup of coffee. I'll add more as I think of it as the day goes on.