You just have to buy 51% of the currency or track down majority of stakeholders and compromise their private keys. Much cost free
lots of people seem to believe this (I think it's even mentioned in Sunny King's PPC paper), but it's not accurate: you need 51% of the actively staking coin-age. That's much, much, less than 51% of the currency.
I had some ideas to help fix this, I'm working on it.
Peercoin PoS is not the same as NXT PoS, the latter doesn't use coin-age.