I think we have all got carried away and completely lost sight of the original problem. If you are trying to fix up an old bike, worth maybe 50 quid, then to suggest fitting it with discs is pretty daft. If you have a bike that does not already have the fixing points and so on for discs, then it is never really going to be a cost effective conversion. New hubs to take the discs, modifications to the frame to fit the mounting lugs for the calipers, the list goes on and on. Better to just go and buy something that already has them fitted.