It depends on what you ae going to pay for the car and how bad you want it. If you are going to build the motor anyway I don't see an issue with it as long as the block is not hurt. I would not run the car very long with the noise that you say it has. What year of Mustang and engine is in it now? If you are going to work on the motor an the price reflects the tired motor than go for it. Can you do the work yourself? That is a problem if you can't and you need to pay to have a motor built. That costs big dollars depending what you do to the motor. Good luck I hope this helps a little. Keep us posted on your progress and decision.
piston slap 
