The main way I make my living is selling horses I have retrained so here are some things to consider. All of these things are based upon my areas market and how good marketing can get you more than the competition.
Don't take on a project you can not afford or have enough time for. If something happens be prepared to take a large financial loss. Timing wise, it needs to be profitable for you. In order to do this, treat it as a full time job. The more you're out there working her, the more training she gets and the more she is worth. Given that you're in high school, I don't think this is a great idea. Between your other horses, school and friends there isn't enough time. Besides that, given your questions, you may not have the experience needed to do this. Given your age, this is most likely on your parents shoulders, can they handle this financially? What if she needs her teeth done, chiropractic work, gets hurt and needs to go to the vet etc?
Why is she being given away?
Take your market into consideration. What type of person are you aiming for? Will she be a trail horse or what?
Take her conformation into a LOT of consideration, tear that mare apart visually. If she isn't built well, she won't stand up to a tough job, be visually unappealing and won't be marketable.
What's her temperament like? If she is good minded, then she'll be a good project to turn into a beginner mount and conformation won't be such a huge deal. If she's a witch, she needs to be built very well because she'll be fit for more experienced riders who know what they are looking at.
How much white does she have? People like paints, but your market is limited if she is predominantly white as they 'aren't as pretty'.
How tall is she? If she's a little bitty thing, then you pretty much have to get her broke enoughto be ridden by a child. If she's 15 hands and stocky, you're golden and if she's 16 hands+ her marketability goes down because then it's difficult to mount from the ground and the size can be intimidating.
What can you offer her in terms of retraining? Can you haul this mare all over and get her used to new places or will you be doing this at your own barn mostly? How prepared are you if she has behavior problems that need to be addressed?
Does she gait consistently? Do you know HOW to train a horse to gait well, or to train it out of her under saddle if she only gaits sporadically and she'd be better marketed as a non gaited horse?