If it actually has a rod knock, whoever "rebuilt the motor" screwed the car up. Everyone else who touched it after that was just an innocent bystander... unless you consider someone who got the car driving again screwed it up, because driving on washed bearings is the #1 cause of rod knock in turbo buicks.
Make sure its actually a rod knock, but if it is, the car is worth $4500 as it sits and nobody in their right mind is going trade a Turbocharged Mustang for it. If it is a rod knock, the motor need to come out and the bottom end be actually rebuilt this time. It's not that expensive and its not like anyone is ever going to make over 400 HP with it, so you don't need to get all fancy with main bearing caps or girdles or a forged crank..