They are going to have a serious problem with immigration. If that border doesn’t stay tight, the north will drain out and the south will have an epic disaster on its hands. That said, much much better than the alternative.
Businesses in the South will love all that cheap labor from the North.
I think there has to be a period where the North can stabilize, before true reunification can take place. It will take at least a generation for the folks in the North to catch up, in terms of skills, culture with the South.