you will need front hub, one shock for disc caliper, and disc brake with complete assembly, if you gor for used one it can be found cheap in Bilal ganj but its a head ache to get it all refreshed and cleaned and thn finally working to best output... but offcourse cheap..
For paiggio parts, Mechni salam or Chita have good sources to almot get any thing of paiggio from the factory but I have no idea about rates offcourse fitting new brake would be a piece of cake and will start working without any cleaning problem and offcourse would have long life.. but its gona be expensive..