If there were to be a united, and presumably capitalist, Korea, I don't see why the Chinese should worry too much any more than they worry about American influence in South Korea.

China is communist only in name. It renounced Stalinism ages ago so I don't see that the Stalin connection between China and North Korea can be a very strong one any more.

Images and videos of North Korea are very chilling and Orwellian. I wouldn't mind seeing it go.