Although Den and Juls dont like rear discs on their setups, I would go with a GEBE rear disc wheel with 105g spokes and keep the existing cantilever bakes too with a double brake lever for the back brakes. My Tanaka 40 runs so fast, I get a little nervous about stopping. (ie... panic stop because the Bozo truck driver just turned the corner and cut you off !! At 30mph, regular bike brakes are not meant for these situations !)
BTW , you will love the dual suspension and your back will say "thank you" for every pot hole and bump. I also installed BMX kiddie handlebars for I can sit more upright in the saddle.