Undercoat the bottom two thirds of the interior, bottom and as well as any "drainage" areas. Keep wind splatter in mind. Be sure to use the hard drying undercoating not the softer stuff. Products similar to RhinoCoat work well.
Paint the top of the roof white.
Pull the doors and add grease zurks. When greasing have somebody else work each door over the full range of motion. If this is not possible use a high pressure PTFE dry lube. Blue Works makes a good high pressure PTFE dry lube.
http://www.blueworksbrand.com/products/high-performance-ptfe-lubricant/Are you going to use stall padding on the flooring? If so, countersink large stainless steel bolts so that they can be easily removed for cleaning and sterilization. Also left over stall padding is excellent on corners, doors, etc, to prevent bumps and cuts to the livestock (and you).
You probably already know this, but make sure electrical connections have a loop down on each side to act as a drip catcher, keeping rain and water away from the connection.
Finally when you are done, ride back there at speed. Listen for noises and check breezes. Horses are especially sensitive to this kind of thing and the last thing you want is pissed off horses every time you unload them. They remember that stuff the next time you want them to load up again also.
