In my mind, you shouldn't buy a horse for dressage.
Rather, find a horse that interests you, a horse that is a good match for you; your skills, personality, size and system of care. Choose a horse whom you enjoy being with, a horse who brings out a sense of pride in you and a horse whose characteristics and gifts you admire.
Then and only then, look to the techniques, practices and pursuits of dressage to grow your partnership.
Dressage should ensure the health, well being, beauty and longevity of your horse and by extension, you.
If you choose to compete in dressage as a component of your journey then use it to explore and enhance your partnership. Avoid the risk of allowing ego to damage your relationship.
To quote one of my mentors, Anja Beran,
"The horse is not for dressage. Dressage is for the horse."
photo taken with Anja at her stables, Gut Rosenhof, Germany
....for those who may still be reading !