The 'stangs that are gentled or started don't require as much specialized containment as a wild one.
Anyway, you should really ask yourself these questions.
1) Do you have the funds for the necessary ultrasounds and various vet visits both before and after breeding?
2) Do you have an emergency slush fund for if/when something goes wrong and you need a vet to come out and assist in the delivery to try to save either mare or foal or both?
3) Has she ever been bred before? 16 is very old for a maiden mare and it's just like with older women who have babies. The older they are when they have their first, the chances for a difficult/heartbreaking pregnancy/birth increase exponentially.
4) Are you prepared for the possibility that you might lose your mare? Having a mare die in foaling isn't uncommon. Can you deal with it if something goes wrong and she dies?
5) Are you prepared for the possibility that you might lose the foal? Having a foal die during or shortly after foaling isn't uncommon either.
6) Do you have a plan for the foal if it has a conformational deformity or gets injured where it can never be more than a pasture pet? What if it's born with clubbed feet or windswept legs or severe knock knees or any other deformity that will keep it from having a happy life as even a pasture companion? Are you ready to make the hard choice to have it put down?
7) Did you know that mustangs don't breed "true"? Because their bloodlines are so mixed with different types/temperaments of horses, you have zero reasonable expectation of the resulting foal's temperament or physical appearance/ability. Your mare is sweet, but she could very well produce the spawn of the devil. I've seen it happen. Dam was just as nice a horse as you could ever hope to meet, but all her foals (crossed on really good stallions) were raging lunatics in spite of proper handling/training.
8 ) Are you really willing to wait at least 4 years to ride the resulting foal? One year of gestation followed by 3 years of pouring money into a horse you can't ride is a very large commitment.
9) Do you have the knowledge necessary to train the foal correctly? If not, do you have the funds available to pay a trainer to teach the foal to lead correctly, to stand for the farrier, to stand for bathing, to load in the trailer, and when the time comes, to be ridden?
10) How old are you? You don't actually have to answer publicly, but I want you to think about all the changes that may be coming up in your life. Maybe you are about to start college or about to start a career or maybe even a family. Are you sure that you'll still have the means to support 2+ horses in 5 years? What about 10 years?
11) What happens if you can't continue to support them? What marketability does the potential foal have? Half mustang? Those are a dime a dozen and outside of certain circles, Mustangs are unwanted mutts that most people wouldn't take home for free.
Anyway, back to her conformation. Everything I mentioned about her front legs especially (clubbed foot, offset knee, calf knees) are considered soundness issues because they predispose the horse to lameness and early onset arthritis. Even if she was a papered QH with the best lines in the world, I wouldn't advise anyone breed her because the chance of the foal ending up with problems worse than she's got is just too great.
You breed great horses and often get foals that end up being just "good". You breed "good" horses and you often end up with a "mediocre" foal. You breed "mediocre" horses (no offense, but her conformation is mediocre at best. She may be a sweetie and a wonderful riding horse, but her body is what it is)....and you run a real chance of ending up with a pretty sorry looking foal.
With her back, where I see the beginning of a sway is in the region circled in red below (first picture). Most normal horses with strong backs are generally pretty straight/flat through that area (2nd picture of my mustang so we keep to the same breed). With the combination of how long her back is and the weakness in that area, it would likely be hard on her back to carry the extra weight of a foal...and may make that sway bad enough that she would require a saddle made specifically for her to fit.