First off, howdy and welcome to the forum
I agree with the grass hay, though I would probably also throw some alfalfa in there just for the added protein. Whatever you feed them, you'll want it to be very fine stemmed and/or leafy. The coarser and thicker the stems are, the more likely they are to develop a hay belly, especially as a yearling.
One thing that does concern me though, how much training experience do you have? Does your mom have training experience? Will there be a trainer involved with this "program" that you can call on if you start having trouble?
Only reason I ask is that trying to train an unhandled horse isn't something to just jump into if you've never done it under the watchful eye of a trainer before. Unhandled horses, especially Mustangs, can get very aggressive if you push them even a little bit too hard and I would hate for you to get hurt.