In the South the enemy is heat, not cold. While the local Chamber of Commerce seldom wants to admit it, the South DOES get cold, but it’s rarely deep cold and it doesn’t last long. In VA you’ll get more than in SC or GA and if you’re in the mountains you’ll get more cold than in the Tidewater. VA is a big state; what works in VA Beach might not be such a good idea in Abingdon.
Keeping the above in mind, take the time to tour some local facilities and see how others have built. The “crowd” is not always wise but if you see a consistent pattern of building in successful looking facilities then you might want to explore how and way they were built that way and then emulate them. Look for signs of damage or other problems so that you can avoid, maybe, what damaged them. Keep your roof line high with lots of ventilation to remove heat from the barn. You can use ridge vents, cupolas, turbovents or some combination of these. Big fans are nice but better you take the time, and spend the money, to use things that don’t require electricity. Windows for cross ventilation are a good idea. Note how the wind, or breeze, blows on the hottest days of the year and orient the barn to be the beneficiary of that air movement. Insulate the roof. I didn’t and that was a mistake. It keeps the barn a bit cooler and doesn’t “rain” condensed moisture.
Make your barn aisle 14-16 feet wide. This will allow you to drive in and load/unload trailers in rainy weather. Position the barn such that you can have reasonable driveway access to accomplish this.
Don’t build a really “tight” barn that will keep air in. You want lots of air in and out. This will mean cooler in the summer and colder in the winter. Horses can handle cold better than they can handle heat.
Build for ease of maintenance. Every minute you spend cleaning, fixing, adjusting, etc. is a minute you don’t have to spend on equine and humine training. Spend the money in the design process to same money in the maintenance area. This is another reason to visit other facilities, as many as you can, and take advantage of the experience of others.
No loft in the barn. That would negate all you’ve done, so far, to get rid of heat. Plan to keep up to a week’s worth of hay in the barn, on maybe in a “lean to” built along it or in an empty stall. Doing this also significantly reduces dust in the barn.
A heated/cooled tack and feed storage are is a good idea. That really reduces the amount of mold and mildew in both feed bags and on leather tack. If you can’t heat and cool at least get a de-humidifier. All this presumes standard electrical service. That’s another good thing.
Water service should be IAW code for your area. I’m not a fan of “automatic waterers” as I like to informally monitor equine water intake for stalled horses. A hot water heater is a nice touch, as is a wash rack that be “enclosed” in cool weather. Then it can be use not only to bath horses but also as a vet/farrier area. You can find industrial heaters that will do in an enclosed 12’ x 12’ space and you’ll be a genuine hero to your vet and farrier. That makes a difference around the time you need their services. 
In setting up paddocks if you have natural water and can use it then make use of it. That saves money in that you don’t have the cost to pump or buy water and means you don’t have to worry about horses running out on either hot or cold days. If you have to pump/buy water then look for the least costly methods of moving it around to where you need it. Consider something like this on your lines out to remote locations to protect yourself against a well pump failure or a really big water bill. http://www.wayscale.com/water-leak-detection/domestic-flow-based
A visit to your local Extension office is usually productive as they have plans and advice for things specific to your area within the state.
There are books written on this subject and these are just few lessons. Good luck as you go forward in your project!
G.
