I use a quarter horse for drill, but any breed will work. You want a fairly average sized horse (no ponies and I usually not taller than 17hh) Most of the horses on my team are around 15.3/16hh
You want a sound, broke horse. He should be able to extend and collect/be easy to rate. He shouldnt have a problem being close to other horses and being bumped into. Should be able to move laterally as well.
You dont want a hot horse or one that is going to think every little thing is going to eat him. The easier they adapt to new surroundings, the better but I think part of that comes from your bond with the horse.