I think that if North Korea ever invaded South Korea (using solely conventional forces, i.e., without the use of nukes), they would still overwhelm South Korea by their sheer numbers. South Korea's military is vastly superior to North Korea's, not to mention that the US would be directly involved too and provide considerable support to the South Korean military, but I think that given the short distance between the two nations, including the proximity of Seoul to the North Korean border, it would be extremely difficult even with the combined military forces of SK and the US to withstand a North Korean invasion. At least at the beginning. The SK and US kill ratio against the North Koreans would be tremendous, but I think they would still overwhelm Seoul and a lot of the rest of SK too. Of course, eventually SK and the US would be able to drive back the North Koreans, but not before they destroyed much of Seoul. Sort of like the Tet Offensive, but even deadlier.
However, I think the only motivation for NK to do this would be utter desperation, a suicidal last gasp to "take as many free South Koreans with them" as possible, as the decrepit and defunct Communist regime lost all its power and authority, and devolved into oblivion.
That's also, as far as I understand, why several of our Administrations (Clinton, Bush, etc.) have done so little in trying to assuage the situation in the Korean peninsula: they're waiting for the clock to run out on the Communist government. Time is not on NK's side.