Sounds great! but be sure to put the horse manure into your *compost pile* first. Unless it's fully composted, the horse manure will contain weed and other plant seeds and may also be too rich in nitrogen, thus 1) sending up competition for your plants and 2) stifling the plants you *do* want due to the excess nitrogen.
But once it's composted and the compost is added to the soil, you'll be a very happy gardener.
Sunset Zone 17, USDA Zone 9