Personally I'd suggest uncapped on that usage, but yes, it does get pricier the faster you get. On capped, nntp and p2p will shape dynamically dependent on network load at the time. Can be next to nothing - can be substantial. All depends on what's going on across the subscriber base as a whole. I prefer to be open about this so you are aware that the "unshaped" myth many ISPs perpetuate isn't really yet a reality here yet considering input costs and the "race to the bottom" price war that happened a few years back. Even on capped accounts, and may in fact become a cause for concern on capped in the future as speeds quickly increase and more peak load IPC is required to run large capped accounts on 40Mbps+ lines. This is the problem with IPC costing in SA at the moment.
So just be aware that someone may come and ask why torrents are not blazing during peak hours on both capped and uncapped (in the current product range) and you'll need to explain to them that it's because Crystal Web isn't yet made of money, but you're trying to help.