Glad to see another member of the B7 forum! Welcome mate!
The 3.0 TDi is a fantastic engine, Darkside are super familiar with it and will be able to extract some serious power and torque for you. Most of their fleet sports the 3.0 TDi, I've followed them closely for a long time. It's the one I would have gone for, but the cam chain setup scared me off in the end...
But if you can get those done, I think it's the best engine of all for the B7, the only exception being the V8 in the RS4. I personally found the V8 in the S4 a little underwhelming, sounds great but slow for what it is. A breathed on 3.0TDi would eat the S4 for breakfast.
Be sure to post plenty of pics on the journey!