South Korea already has plans in place for that. I can't find the article at the moment, but they feel super bad for and about North Korea and really want to help if they could
Not a very literate and informed reply I'm having here, but just from friendly conversations with a few South Koreans, what I'm getting is that the current generation, many of which haven't grown up in an active war situation, have a much smaller appetite for reunification than their elders may have had. And while they have plans in place (there's a Ministry of Unification) I doubt it that South Korea has money set aside for the real cost of unification (since those amounts would be so tremendous that it'd probably be economically and politically irresponsible to withdraw that much money from the active economy for this long). But then, I'm as little an economic expert as I am an expert on Korea's reunification...
I spent some time in South Korea working with several churches there - they are in tears over the situation in the North every day praying for them. I'm sure if/when it happens there will be a staggeringly huge amount of money poured in from the churches, and other religious people around the world (on top of everything the governments put in...)
Everything I've read about a postwar reconstruction of North Korea says that the costs would be much, much greater than the integration of East and West Germany.
Not only assuming that a hot war would have serious consequences to the economy of South Korea.