If you need to keep things really light (although the car seems heavy), then you should look to bicycle disc brakes. The issue with creation of the mounting studs will be interesting, as will the method by which you attach the rotors to the wheels you use. If you use bicycle wheels, already, just use disc-hubs and then look at getting some 8" rotor, hydraulic brakes, such as those made by Hope, Hayes, Magura or Avid.