Well the Dollface certainly is not underweight - lol lol She has a beautiful face and is my kinda color
I'd be willing to guess she's more like 750 - 800 pounds if she's 13.2H
Just to let you know, Morgans and TWH's are on that nasty list of breeds called "Predisposed to Metabolic Issues", so you will have to be exceptionally careful of her weight and what she eats.
The lower you can keep her starches and sugars (hay and pasture also get factored in), the better.
While oats are a good mainstay for a horse, they are not good for easy keepers and horses that might be predisposed to metabolic issues.
Were she a QH, I would keep quiet on feeding oats but, given her genetic predisposition to metabolic problems, I would stay as far away from any sort of grains as I could. Legume hays, such as alfalfa, aren't good for these types of horses either - at least not until they hit their 20's and old age dictates they need more protein and amino acids.
I like your feed guy; I have a feeling he and I are close to the same age - I'd never get out of his feed store