It would help immensely to have a trainer to help you along in finding your horse and in ultimately helping you reach your goals with your future horse. At the least it would be extremely helpful to have at least some very experienced horse person to help you search and sort through the horses you see. As you continue looking online and elsewhere, look for horses that are described as well-trained, great for beginners, experienced, etc., with calm temperaments and go out to see a lot of them. If you are serious about buying it is always best to look at and test ride as many qualified horses as you can to figure out what is really best for you.
I'm not familiar with your situation, whether you are experienced with horses and just looking into a new discipline or new to horses, but either way you want to be sure to get a horse that will already know his job well and not need you to try to train him while you are being trained in this discipline, yourself.