Assuming it's flat bar I'd buy whatever Shimano hydro brakes you can find on sale. The British sites usually have the best deals but it's a bad time of year for a deal.
If you want to stick with the mechanical then BB7s are the way to go. You should be able to reuse everything else and just swap the BB7 calipers. New cables and compressionless housing is a good investment.
If you are mountain biking, go for the hydros.