In SC you are supposed to get a TFE (Temporary Food Establishment) permit and won't be allowed to cook any food until you have been inspected by the health dept. Contact them to find out what you need. You can bring in precooked food but it has to come from a licensed commercial kitchen or commissary that you name on your permit application. Bringing it in cold and reheating is a no no.
I have competed and vended a few times and done ok in comp but always had a staff take care of vending while I concentrated on comp cooking. $5 a sammage is pretty standard but sometimes you can get $6.