If you have rescued horses and you're feeding hay on the ground, the horses are going to eat a lot of sand with the hay. If their bodies are stressed, they're not going to do well with sand impaction colic and your vet bills will be high. If drainage isn't a problem, then I would go with wood or rubber chips as a good base for their joints. Pea gravel probably would be gentle enough on their feet, but you can probably buy a lot of wood chips for the cost of pea gravel. Of course, you could also concrete under the haying area.