Um...you could do all, really...if you find a horse who is well built, essentially you could do anything with it. And WP and dressage would be easy for a nicely built horse. I've always had horses who were pretty good at "all around"; Did pleasure classes, some jumping, as well as high adrenaline gaming...all on the same horse.
SO, my advice would be to maybe find a horse who has training in either dressage or WP, and take lessons, and train the horse for the other later on. If you are focusing on WP, then find a horse who already has some WP training...remember any horse can do dressage, he may just not reach the grand prix levels, but then again, "most" horses won't anyway!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."