I don't think there's a way to completely preempt the North Korean artilery from launching at Seoul. The question is how effective such a strike would be. Either way, I can't see Seoul getting obliterated as it would happen twenty years ago.
North Korean military poses a serious challenges in two areas: nuclear weapons and possible prolonged guerilla warfare. Conventional forces alone would be dealt with quickly.
I'm not sure what is your military credentials, but the DPRK could blitz forward with their shock armies in the old soviet fashion and cause quite a lot of havoc only to fall back to hard built defence lines on the DMZ. They don't have the materiel for a prolonged campaign but I'm not sure the ROK would be willing to sustain this amounts of damage to topple this failed state (only to pay higher prices for the restoration of proper society there).
