In PoW currency you can remine a block to build a longer chain. In Nxt the order of generating accounts is determined, you can't create a long chain that contains blocks generated solely by you. With 51% of the stake the odds to generate a longer chain with 10 blocks are less than 0.1%. If someone buys a car with NXT they can wait a little bit longer to counteract even 90% attack.
I like that you plan to make things deterministic. My ideas for pure PoS have also been deterministic.
There is a potential problem here though.
How do you deal with AWOL coin-owners? If I can't make a winning chain with 51%, then can the chain continue at all if 49% of coins are lost?
Looking forward to the details so I can see how you address this and other issues.