ou're right-- getting from Seoul to Boseong is not super feasible, but I would say that even from Busan, it's a little bit silly. Like someone else in this thread, I'd suggest staying in Suncheon or Yeosu. In Suncheon, I know there are a billion guest houses near the train station, and lots of cheap love hotels you can stay in. But you can find those in ANY city.
As for transportation, I found that getting to Boseong via city bus is really easy to get TO the fields, but getting FROM the fields is near impossible. If I had to relive my Boseong-ing experience, I'd take the bus there, and call a cab to take me back. Cheaper than renting your own car.
There are a lot of other things to see and do in Jeollanam-do, especially if you only have a few days. If you stay in Suncheon, you can check out Suncheon-man (Suncheon-bay, which is a kind of reedy wetland area. Pretty nice, and there's a tiny hike you can do to get a good view of the area) and Naganeupseong (I probably spelled that wrong) folk village is a nice cultural thing as well. I know less about Yeosu, but they have a really kickass aquarium, and lots of other 'touristy' places to see. Lots of islands and beaches. Also in the area you could go to Namhae (beaches, German village, cute B&Bs to stay at), hike in Jirisan (big area, but if you're interested, I know of a great hike and temple to see here)...
Don't limit yourself to Seoul/Busan! Even if you decide to just stay in Boseong for a night and head back to the city, though, my main advice is to take a bus TO the plantation and then call a taxi to come back.
Good luck!