TC,
I believe the most reliable source for almost all English kings if you want to know what they looked like are images of their Great Seals. A Google image search will provide you with a couple of options. I rationalise that all kings were intimately involved in how they were depicted on their royal seals and that it will be an accurate portrayal of them in their war gear as they were at the time - including the horse furniture detail. John is not quite so flamboyant as his older brother.