you're going to need to check with the town to make sure horses are allowed at residential homes, and then see how many acres per horse you need. Some towns it might be 1 acre per horse, other's it might be 2 acres per horse.
Along with a farrier, you're going to need to have a vet. You're going to have to find a place to get hay, shavings and grain. You're going to need to store the hay shavings and grain also.
I'm not 100% positive on this, but you might have to take the plan to build a stall to zoning to get it approved. Again, i'm not sure on this.
Obviously along with having the horse, you're going to be in charge of everything! Also think about when she needs her shots. Are you going to have the vet come out every time she needs her shots?
Since horses are herd animals, they always do better with another one of their species. She could get lonely and/or bored, especially if you're in school and have other activities.
How much time do you think you'll be able to devote to her?
For Manure, i highly doubt you'll be allowed to leave it in a garbage bag for the garbage men to take. You'll have to either get someone to remove it or compost it.
I would also suggest you find out the last time she was wormed and with what wormer and how much she got. Find out what feed she's on and decide if you want to stick with the same feed or change it. If you change it, slowly introduce the new feed over a 10 day period would be good
Same thing goes with the hay. If you change too quickly she could colic.
Since you did say that you're going to stay with your aunt over the summer, and then return Chance when the summer's over, i'd be concerned that she'd get very stressed from moving all the time. Each time she changes places, she'll have to get used to it all over again.
Depending on if you want the fence to be permanent depends on what type of fencing you do. The metal fencing would be good as it is also easily able to move and store.
Wood fencing would be more permant, but if you're not going to keep Chance there all the time, i guess it wouldn't matter.