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.
it doesn't matter, it has the same probblem, just replace "coin-age" by "coins".
form their "whitepaper" (actually a wiki):
tokens must be stationary within an account for 1,440 blocks before they can contribute to the block generation process
this means all coins that are used for transfers cannot be staken and do not count toward the total of which you need 51%
moreover, lots of holders do not stake, so it's not 51% of coins, it's 51% of coins being actively at stake