just thought I'd throw my experience in here-lived in Seoul and Gwangju for 2 years as English teacher (2000-2002). Bought a used daelim 125 sportbike and rode it all over the country-as far north as the outskirts of Seoul, as far east as Busan, and onto Cheju-do for a tour.
The Korean 125's are great for the city: light, nimble, cheap to run and fix and easy to get parts for.
For out of city riding a bigger bike would be better-lots of r1's, gsxrs, busas, harleys, wings... when I was there.
Knowing the country the way I do, and the kind of riding I like to do (twisty roads, trails, beach tracks, farmroads... if I could choose any bike to take, a 400-650cc dualsport with 50/50 tires (tkc80's) would be it.
I'd buy my gear there.
Why they aren't allowed on highways? : a 125cc tops out at 135km/h on flat ground in full tuck-traffic runs 120-140km/h. At speed you can't tell how big a bike is, so it's easier to just ban them all-at least that was my take on it.