I don't believe that the North Koreans are serious about going over the DMZ right now. For one thing, they just held the Korean Workers' Party Central Committee Plenary Meeting in Pyongyang. If you are preparing for war, you don't centralize your leadership in one place and paint a big bullseye on it. The second reason is that, while the report of the meeting emphasized the North's commitment to nuclear advancement, it also covered a number of mundane tasks, such as promotions to vacancies within the party. The coverage of routine business implies a lessening of tensions and a return to business as usual. The analysts at KGS Nightwatch believe that North Korea's motive isn't an immediate war, so much as our recognition of the North as a nuclear power. North Korean threats require responses and contingency plans by the US and South Korea to fight a nuclear-armed North Korea. Even if the North lacks the capability, the threats demand some precautions by us, and these provide Kim Jong Un with de facto acceptance of North Korea as a nuclear power. Basically, they figured that the only way to force our acceptance of North Korea as a nuclear armed state was to threaten a nuclear war. It worked. The joint exercises were a good excuse for the saber-rattling, but the real indicator will be if the North retains the war footing through April, when the spring planting must begin if they are to have any harvest this year. If they don't plant, then they are serious about their endgame.
As for the North's capabilities regarding the DMZ, they have sufficient materiale and ordnance to level Seoul, but they cannot advance through the South, as the retaliatory strikes will destroy them. Their equipment is no match for what South Korea can bring to bear, and they lack the depth of logistics capabilities to sustain combat for more than a few days. The North can start an ugly war, but it cannot win one, and they know this. China is the North's only regional ally, and it will not support any action that impacts the stability of the region (the last thing that China wants is increased American military activity that close to home), and their other allies (Iran, Venezuela, Syria) are in no position to provide them with any aid. This is all bluster.