The temperament + the bone + the size (Small). The color is a bonus. If size wasn't an issues I'd just buy any well conformed draft cross.
Ok. If it were me, I would look more towards the welsh cobs over the gypsy as they will give you everything you are looking for except color and are known to cross well with Arabs. They have been breed for hundreds of years for bone, size and temperment. Even the queen of England rides them and it would make the horse (pony) valuable as a child's mount if anything ever happened to you.
If you are dead set on a gypsy for the color, look for one that is a high % of welsh.