Dr. Diff is the only way to go. Cass (owner) is fantastic. I would step up to a 17" wheel and run the Cobra setup front & rear, but here are the 'budget' minded kits for both wheel sizes.

17" wheels -
http://www.doctordiff.com/cobra-style-13-front-disc-brake-kit.html
http://www.doctordiff.com/rear-disc-brake-kit-11.7-rotor.html

15" wheels -
http://www.doctordiff.com/front-disc-brake-kit-for-11.75-on-disc-knuckles.html
http://www.doctordiff.com/rear-disc-brake-kit-10.7-rotor.html


What you're going to need -
http://www.doctordiff.com/aluminum-master-cylinder-kit.html

http://www.doctordiff.com/reproduction-disc-brake-knuckles.html

http://www.doctordiff.com/adjustable-proportioning-valve.html

https://www.amazon.com/gp/product/B015YGREM6/ref=oh_aui_detailpage_o09_s02?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B01KNGVB6W/ref=oh_aui_detailpage_o09_s03?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B000BHQ1CS/ref=oh_aui_detailpage_o09_s03?ie=UTF8&psc=1