Well, she is obese to the point of I would get blood work run on her to rule out health issues. Has a vet looked at her recently? Is there a PPE being done?
She has a very short neck, is goose rumped and has a long back. Can't really tell much else due to the severity of her obesity.
I would focus on getting her healthy before riding her too vigorously as overweight + out of shape = injuries.
Would be interesting to see her healthy and what her confo is like then, but from what I can see now I would not expect her to do well at dressage, even at training and first levels. She might work out to be a great recreational and trail horse, depending on her temperament.
They say money doesn't buy happiness -- well happiness doesn't buy horses!