Germany ? Vietnam ?
Not many others.
Vietnam case is an interesting case if you want to compare. In both cases, the state is a communist dictatorship which depended from USSR help.
Vietnam got much more bombs that North Korea, much later and in 1980, Vietnamese economy was in a much worst state than the North Korean one. Vietnam reformed its economy in 1986 and today it is one of the fastest growing economy in the world.
Today, Vietnam got many problems, corruption among them, but between Vietnam and Korea, the choice is easy.
Anyway, in 1970, the effects of the bombing had been overcome. What happened after is strictly caused by the North Korean government inept policies.
And to answer the first question : What will it take to make South Korea and North Korea into one single country?
Only one thing: that North Korea become a democracy.