Advice on visa and marriage in Korea?

Hello, I plan to go to Korea next year and I need details on the visa/citizenship/marriage process for a US citizen with a Korean citizen. My plan is to visit first to get myself pre-situated and return to US once before making the final move for Korea. What kind of visa do I need, or do I need one to just visit for a few months? (What is the number of days allowed without a visa?) If I decided to just stay in Korea without returning to the US the first visit, would I be able to apply for visa while in Korea? Or would I have to return to the US and apply? I am already aware that I can get a dual citizenship through marriage. And I do want him to have a dual citizenship as well, so we could have an option to live in Korea or the US in the future- have any input? Thanks so much in advance.