I bought a repalcement fork with disc mount and left the rear brake alone. Surly has a good selection of 1 1/8" threadless disc forks. If you bike has a 1" threaded fork, I'm not aware of any options. If your headset is 1 1/8" threaded, you'll need a new headset too. It'll run at least ~$200 total, you really need to love the frame to do this modification. A new/used bike with exisiting disc brakes is usually a better option.