The big thing that makes her not breeding level and will affect her a bit (no Grand Prix dressage, ok?) is that her neck is really short compared to her body length, is set on low and is way under muscled. The muscling can be changed, the small/short neck cannot. Trim up her mane and it will help make her neck look longer, give her a hunter look.
I think you totally lucked out. It couldn't possibly be that her calmness is due to malnutrition? But, no, since you've had a her awhile.