Good find. Here's the scoop from Auto City Classic (located in MN, I talked to a guy named Denny, very helpful).
The front disk kit is $649 plus about $40 for shipping. It includes the master cylinder, proportioning valve, 11-inch rotors, single piston calipers, pads, hardware, etc. Just about everything you need except the lines that run from the proportioning valve to the hose connection. You'll have to fab those yourself.
The calipers are re-built calipers, typically from '69-'72 or '78-'87 Chevelles, so replacement parts (pads, seals, etc.) are easy to get if you ever need to replace anything.
The also sell a rear disk conversion kit that utilizes rear rotors from 80's-style Seville's. It costs $499 plus shipping and comes with a proportioning valve for a front & rear disk setup.
Both kits are in stock and delivery time should be less than a week.
They also sell windshields for C1's ($189 + $85 shipping for non-date coded glass and $269 + shipping for coded glass), gas tanks ($169 + $25 S&H), and power window kits.