I would look for a car that hasn't been tuned. It's probably less likely to have had the hell driven out of it. If the seller says it hasn't been tuned, one way to check may be to have your PPI done at a Ford dealer. My understanding is that they can tell how many times the car has been started since the ECU has been reflashed, or reprogrammed. If it's a low number, this means that the ECU probably was reflashed recently. One possible reason for this might be the removal of a tune. There are others, but if that's the case I would try to find out why it was reflashed.