If your bike didn't start with a disc brake, conversion setups are expensive (new forks, calipers, pads, rotors). If you can find a decent used wheel with a drum already laced in (check ebay) then its going to be a lot cheaper than converting to disc. Even a new wheel with a drum is probably cheaper than converting to disc. Also probably cheaper than buying a whole new bike just to have disc brakes, especially if you're going to need to upgrade anything right away (bigger rotor, more durable pads, etc). I run a front drum and rear coaster and go 30-32mph. My good buddy jbcruisin has a bike that goes almost 50 mph and runs the same setup. You won't need a new fork to install a drum brake, as you can simply fabricate a bracket to hold the brake arm in place. The only thing you will need is a brake lever with good pulling force (i.e one that curves outward away from the bars a little). A standard v-brake type lever should be sufficient. I use a motorcycle type lever, but mostly for the built in brake light switch. I had to bend the lever out a little to get the amount of pull I wanted. Luckily it was made of aluminum. I very gently bent it a little at a time with a seatpost until I had it where I wanted it.