Very wise choice to convert your allroad to 7 seats :) iv got a regular A6 with the feature.
Not meaning to tell you how to suck eggs but you'll need rear seats, seat belts and mounts, floor mounts for the seats and belts, boot side trim which includes cup holders.
I believe the seats were only available in light grey cloth and I know the cost new for the conversion was approx £1700
in the pics you should sea the seatbelt part numbers
hope that helps :)