Middleton has several different locations where you can have the ceremony, including my personal favorite, the Wedding Oak:
Nothing says "Lowcountry Wedding" like exchanging vows under a massive live oak tree!
Or, if you prefer, you can marry on the front lawn in front of the main house (sheep not included). I saw a Charleston Weddings magazine article on a couple who had a destination Great Gatsby-themed wedding on the front lawn. Amazing!
Another option is Drayton Hall, the only one of the "major" plantations in Charleston that hasn't been completely restored. Although you cannot have your event inside the plantation house (as it's completely bare--no a/c, no lights, and in many cases, no windows), you can rent the entire property, set up a tent and have an incredible background for your ceremony and/or reception. If you'd like to rent the property just for the ceremony, it's $10,000, and you can have the ceremony and reception for $11,000. Chump change, right?