I have seen estimates and studies on what would happen in a conventional invasion.
North Korea would open up artillery fire that will be able to reach Seoul, along with maybe missile attacks that might be targeted at some of the larger cities. These missiles might be taken down by Patriot missile batteries, maybe not. Seoul will suffer massive damage and casualties.
N. Korea has so much of its firepower and men parked right along the DMZ. The only possible way to decapitate them would be a tactical nuclear strike,which would be stupid cause of the fallout that would spread to Seoul. Hence, before the S. Korean military and U.S. can counter, much of Seoul will get destroyed.
But once the counterattack starts, it will be massive and swift. South Korea's miliary is one of the best trained in the world. The U.S. is obviously the best. Massive air strikes will take out much of the artillery battery in the North, along with missile cites. Strafing runs will destroy much of the enemy troop capability. South Korea's tanks are similar to the Abrams tanks of the U.S.
Most predictions are that North Korea's soldiers will not even make it to Seoul, before being pushed back by South Korea's military, with help from US airpower. Eventually, commando runs will occur behind enemy lines, f-ing up their command structure.
Then the plan calls for counter invasion and regime change in North Korea.