For a rear wheel, pretty much any brake should be sufficient (for all but the messiest of commutes*). 80% of stopping power is from front brake.

All that said, if you want, just get a Nuvinci that is disc brake ready if you don't want rim brakes.

* If you do switch to rim brakes, getting the right brake pad can make a huge difference here.

p.s. For front brakes, I think disc brakes have a lot of merit. For rear brakes, the biggest reason to have disc brakes (for most riders) is because that's what you've got on the front.