Firstly for the size of their army they have ~1,280,000 active personnel (~4th, between US and Russia), 600,000 reserve and 5,700,000 paramilitary (overall 1st in the world with 7580000 personnel).
They have 5845 tanks (2nd in the world to Russia) and ~11920 artillery (2nd again to Russia)
They have 35 submarines (4th to Russia, USA and China)
The short border will be deadly, expect Seoul to be fully destroyed.
There’s no way the USA and/or nukes won’t get involved if Seoul is fully destroyed. Seoul and South Korea in general play a (somewhat) major part in the world economy, at least compared to the DPRK. There’s no way that the majority of the world’s countries who trade with the ROK will want them to fall to one of the most economically isolated countries in the world.
