You really have to see how the dog and horse will get along. Chance might view the dog as a predator (which in nature they are) and the dog might see Chance as prey (which in nature they are). And then any accidents that might come with it, Chance kicking the dog, the dog bitiing Chance. Is that worth it? I would also highly recommend that you supervise any time the dog is out with Chance, just for safety, but that's just me.
A goat or donkey or something would be a good companion as they could both romp around together.
Does your Aunt have any vet or vet tech experience? If she doesn't I highly suggest asking a vet to come out and showing you how to administer a shot. If done wrong you could really bad side effects.
Would you have your Aunt help you out if you had to work? Wasn't she the same one who wanted to keep a horse at her property but only 1/2 the backyard and you highly protested it? Now I know she'll have the full backyard, but that was just something I thought about.
All because she's moved a lot in her life does not mean that it's not stressful on her. Wouldn't you like to just find a place and stay there if you moved around a lot? Not to mention a horse has to adjust to their new place. Could take 1 day, could take 1 month or more. And the fact that she's going to be going from 175+ acres with a lot of horses down to about 1 acre with no other horses, that's a lot
to adjust to.
When were you thinking of moving her to your Aunts?
When were you thinking of moving her back to your current place?