LLC or S-Corp for U.S. citizen living in South Korea
From your description I think an LLC in Wyoming would work perfectly for you. LLC is flexible as far as taxation goes, so when (and if) your business is ready to be converted to S-Corporation-based taxation you will be able to elect your LLC to be taxed as S-Corporation.
From what I know multiple-member LLCs are considered to be better protected as far as limited liability goes, but even a single member LLC is still a good tool in separating your personal assets from the liabilities of the business.
You are welcome to read more about forming an LLC in Wyoming here.