Anyone have a recommendation for a disk brake system for use on front hub installation? My experience with e-bikes has been so positive that I am getting ideas for transforming my Trek Navigator 200 into the "ultimate e-bike" (for me). I plan to use 500+ watt front hub motor with a selectable voltage setup (36v to 72V). Am concerned about the braking performance of rim brakes. Would also like to install rear disk brake but that would appear to be more straight forward on first glance. OR IS IT?

Thank in advance for your help!

If your Navigator does not have disk brake tabs on the forks, then you're out of luck. I wouldn't worry, most heavy duty rim brakes, like cantilevers and V-brakes are more than adequate for an E-bike.

The eZee kit includes a disc brake.
http://ebikes.ca/ezee/

The torque you are generating with braking could be bad for your drops. Do consider using a rear hub, with a disc up front.