Slight issue. With POS (at least in an implementation such as NXT), by the time you bought up enough of the coin supply to "attack" the network catastrophically, the only holder affected significantly by such an attack would be yourself
Put another way, it would be like buying somebody's house off them so you could burgle it
But again, the interesting thing is that you can buy up 51% of a PoS coin, and then sell off your coins so that you no longer have 51%, but your history of having once owned 51% makes it possible to attack the network at any time, for free.
So if there is ever a PoS coin with >51% ownership at any point in its history (even in just the Genesis block) then it is vulnerable.