I would never buy a used radiator unless I KNEW how old it was and if/when it had the fluid changed. If it has metal end tanks you can have it rebuilt for ~$100. Get yours rebuilt or get the new Nissen. To answer your previous question: the impellar on the water pump can corrode and/or start spinning freely on the shaft.

Also, if you are replacing the radiator and water pump, certainly replace the coolant cap and thermostat. Also consider replacing every single cooling hose.