All of the advice I have heard indicate that it is best to buy a new one, and not a rebuilt one. I know of several people that have had rebuilds go bad within 12-18 months. Even if they have a "lifetime warranty", you still have the mess and incovenience of replacing it again.
The GM unit is expensive, even if you get it from Dal. When I needed one a couple of years ago, I ordered a new Raybestos unit from Partsamerica.com