Never underestimate S Korea military. I believe S Korea military today has enough power to beat N Korea, S Korea also has well developed conscription like Israel and Singapore. There is a quote that I somewhat remember, when somebody ask someone from Singapore, how many soldiers can tiny Singapore muster, the answer is 2 million soldiers, including the conscription. S Korea is about the same. Of course if war happen with N Korea, the cost will be huge to South.

Anyway, back to the topic. Of course we can't assume the US will leave Korea, if Korea is unified under South. My point is that, if the US leave, good, if the US still stay in unified Korea, then it will be not much different than today. But there is chance that South Korea will force the US to leave if Korea is unified since South Koreans generally don't want to have hostile relationship with China if their sovereignty isn't threatened. One thing for sure, S Korea will not ask the US to leave as long as N Korea is there.

IMHO, China should strive to be powerful and wealthy nation, but not seek hegemony, especially not through military mean. Seeking hegemony, especially through miltiary mean will only force Korea and Japan to invite the US in, or worse, go nuclear. China can still be "respected big brother" through economic means though. Build your hard power, but then build your soft power on top of that and use it to influence your neighbors. Your neighbors will be more inclined to listen to you.