Here's what I would do.
First, get your vet out. Have him give her all needed vaccinations, float her teeth, and remove any wolf teeth.
Next, deworm her thoroughly. I would give her TapeCare+ Plus now and Ivermectin in 4 weeks. I would use Equimax or Quest Plus 8 weeks after that.
to keep track of health and performance records online, free! Plus it will send you reminders when you set due dates.
For her diet, I would recommend she get as much grass hay as she can eat, at least 25 lbs a day.
For her "grain," I would recommend one 2qt scoop of shredded beat pulp (soaked, excess water drained off), one 2 qt feed scoop of Alfalfa hay pellets, 1/2 of a 2qt feed scoop of Whole Oats, and 1 cup of stabilized rice bran or 1 cup of whole flax seeds. The beet pulp is good for weight, though optional if you can't find it. If you don't use it, add some water to the hay pellets, so the rice bran and supplements have something to stick to.
Add to that the following SmartPak (prices are monthly costs): Focus WT
1 scoop $17.60 Mega-Cell™
1 scoop $8.05
The FocusWT is a probiotic that will make sure she is digesting all of the food she is eating properly. The Mega-Cell is an affordable vit/min supplement that will round out her diet. I have found that horses gain better on a more "natural" diet, without all the sticky sweet feed or high starch pellets.
My "hard keeper" TBxArab gelding now gets fat if I'm not careful on just 1/2 scoop of alfalfa pellets, 1/2 scoop of oats, and his vitamin supplement, lol.