I got a "pet quality" Akita that was a breeder return just because I agreed to give her a good home, she was with us for 9 years. She was about 4 when we got her, and had none of the puppy energy that could be so taxing for an older person. Old dogs need good homes, and are usually pretty easygoing as long as they get their kibble and a place to nap, and usually have a good base of housetraining and obedience to build on.
Her replacement is a lab that I got trained, but young. Energy level was definitely something to be reckoned with for a couple of years until she settled down.
We have an Aussie we got as a puppy, and he is such a character. Similar herding drive to the cattledog that we had before him, but more family oriented, not such a workaholic obsessed dog (he had a job, fetch, and he took it very seriously). Aussies have such large personalities I can easily see that if your Mom has bred them that anything else would just not be the same.
So I like the idea of getting an older trained/mellow dog for her, and letting her know that if she gets a dog that it will have a home with you even if she can no longer take care of it (assuming of course, that you are willing to do that). And of course, an Aussie would be best because it is what she knows and is used to. Perhaps if you feel strongly that she should have a dog you could work the rescue angle, that "this dog really needs a good home" and playing down the expense. Make it more of a two-for-one deal where both she and the dog need what the other can offer, not just her needing a dog?
Yes rescue dogs are expensive, but their fees have tripled in 10 years pretty much because vet fees have also tripled in that time. Between fees for services, lab work (no pun intended), and 6 months of heartworm/flea meds it can get pretty close 400 a dog when I walk out again. I have no problem reimbursing a rescue for vet fees, vaccinations, and fostering a dog to make sure it is adoptable, as long as they are honest and up front about health and behavior issues and are working in the best interests of the dog (not just trying to do anything they can to get the dog into any house that will take them/out of their system). Sadly, I know those guys are out there too. I have had good luck with breed specific rescues, for what that is worth.