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!