Learn before you buy!!!
I suggest you find Dr. Deb Bennett's books on Conformation analysis. Take a look here: Equine Studies Institute Homepage
Conformation is physics. The laws of physics do not lie to you! If you can get the physics going and the right horse with the right 'tude, you will have a match. It is not easy!!!
The horse that will do high end dressage will have different physics requirements than a horse doing barrels. Some of it will cross over (short back, proper shoulder and so forth) but some things will be less or more important depending on the discipline.