Questions about similar classes and dressage?

I would get the horse that is trained it what you don't already know, it will be harder for you both to be learning at the same time than if the horse is already trained it in and you can just focus on yourself. So then you can train the horse in what you already know. I would get the horse that is already trained in the thing that is hardest, that way you can just train it the easier stuff (unless you want a challenge of course). I would take lessons if dressage first, you may find that you really don't like dressage at all.