These rads are so cheaply make, and the consequences of a failure are so bad, I would not screw around with a used one. Mine never leaked, but the outlet pipe broke off, spewing all the coolant out. Of course there was no coolant left to set off the temp gauge until it had blown the head gasket and warped the head. $3k later, the car runs great.
My advice: replace it at 80K even if it looks good.