It depends upon what type of trail work you plan to do, just as what you look for in a race horse depends upon what length races he will run and if there are jumps involved.
If you are planning long, hard rides over rugged terrain, then you look for hard hooves, strong bone, a short back, a thick strong neck, a muscle composition conducive to both endurance and strength, a deep girth, and a disposition that is both quiet and trusting.
As you move toward shorter and flatter work, the ideal qualities above become less and less necessary, until you get to gentle trailwork, that almost any horse can do...