This is profoundly underselling things. Seriously, German unification is still not done, and East Germany was a model state about as good as the Soviet bloc could make it. Never mind that Germany literally never, even immediately after WWII, had it as bad as North Korea as far as qualified educators, bureaucrats, judges and all the other people who make a state run. Are there actually qualified dissidents with strong enough credentials to rebuild the systems? Do they actually have what it takes to give the students in their systems a modern education? Even with a heroic effort that would badly damage South Korean educations by cannibalizing their society for everyone who can teach for a long time, I doubt that's the case. Quite bluntly even if the North Korean military hangs it up and goes home and everyone in the government does everything according to an optimal plan in a perfectly orderly fashion, this is going to take generations to fix. Because of language barriers, South Korea will have to buy whatever future they want for North Korea with their own future, and that will be neither cheap nor fun.
That's just the future, and why kids who haven't been born yet would still be screwed and left unable to contribute properly to a modern economy, leaving their area impoverished. There's a lot of people now who are barely going to be able to function in the modern world. They don't have an education, a ton of them are suffering from the effects of malnourishment, it's painfully obvious that there's a lot of people who will have to be given make-work if employing them is desired, or just given welfare so they don't die.
But that's not what'll happen at all. The North Korean army won't give up, or even if they do, they'll change from an army to a bunch of desperate people with guns in a country where survival is uncertain and the basic functions of a state aren't reliably there. Seoul is going to have a lot of damage, even if the worst of it will likely only be some of the city. There's going to be a lot of costs from any military action necessary, and the North Korean state is going to be in horrible shape, and the politics outside there will not guide towards the easiest or best way of dealing with things.