Just about any horse can preform in Dressage, so long as they have the mind for it.
How high do you want to go as a Dressage rider? Most horses are able to preform the lower levels of dressage. Some are able to preform the 'medium' levels, and few are truly able to advance to the upper levels. The mental stress and physical requirements are something some horses just cannot handle, at which point they completely break down.
When looking for a Dressage prospect, I look for a serious horse. One that doesn't have his head in the clouds going "LA LA LA GRASS LA LA BUTTERFLY LA LA LA OMFG CAR -panic-". Instead, one that has focus and a firm mind. I want a horse that will learn and work with
me. I certainly wouldn't want to put a whole lot of training into a horse that then decides he'd rather be a trail horse, or possibly a fairy.
Physical characteristics are also important. If your horse is downhill, sickle hocked, pigeon toed, etc. perhaps you shouldn't do Dressage. It is beautiful, but very physically demanding and can do more harm than good.
Movement is what I look at last, despite it being very important. The horse should flow at liberty, maneuver with grace and elegance. A horse with a very short or wonky stride isn't one I'd want to undertake as a Dressage prospect.
I adore Dressage myself and love watching the horses dance. A horse that loves to dance is simply an awesome thing to watch. It's pretty easy to spot a horse that is meant for Dressage, versus one that is simply trained in Dressage.
This is also a great resource: Dressage Training Pyramid
Your girl is really adorable. Have you done any Spanish Walk training with her? She certainly seems like she has the ability