Without nukes, they pretty much beat the south the first time before we stepped in.
They were a lot less hungry back then, and in terms of weapons, were evenly matched with the southerners for the most part.
I agree however, we should be cautious of underestimating the Norks. They have nuclear weapons, and while logic would dictate only the lunatics in the Muslim world would detonate nuclear weapons today because of MAD, the North Korean state is very fanatical, and are just the sort of people who when losing, would take everyone with them.
The first time the north was industrialized and the south was agrarian, That is not the case now.
You know how the Union had the lion's share of industry and then some prior to the start of the Civil War? The Japanese who ran Korea for 50 years after annexing it in 1895 put most of Korea's industry in the north. The North was also handed a bunch of Soviet surplus equipment from WWII, whereas the South did not even have artillery - the State Department was worried (probably correctly) that any heavy weaponry would be used to invade the North, and made sure that the South was armed mainly with light weaponry, meaning they had no howitzers, tanks or aircraft. Today, the tables are turned - South Korea is a developed country on par with much of the West, whereas North Korea is economically worse off than most African countries. In terms of total national output (GDP), South Korea's number is almost 100x that of North Korea. This is why it's pretty clear that North Korea does not have what it takes to overrun the South.