The problem you will run into with VAG is that often times things look very similar, but are not exactly the same.
There is no such thing as an A6 (I hate the "mkX' moniker) wagon. The A5 wagon got a mild facelift and an updated dashboard. It is still very much the A5 underneath all the way through 2014.
That said, the A6 (which for us, means ONLY the Golf from 2010-14, Typ 5K1) is not drastically different than the A5.
The A7, while still an evolution of the AX cars, is more different yet. But again, some things change a LOT, some things change only a little. And given VAG is much like Lego, lots of parts from other models and other platforms can be used on something else.
For instance, on the NCS Jettas and NMS Passats (which are both similar in many respects), if you want to you can swap in the all-aluminum front subframe to replace the cheaper, heavier stamped steel versions those cars got. So a B6 Passat, or the CC (which is mechanically a B6) can be donors for that subframe.
You can take the fancy cast aluminum front carriers from a newer Audi A4 or A6 that still had the top mounted steering rack and put them in a B5 Passat to replace its heavier cast iron carriers.
You can swap over the first generation Audi TT's front steering and suspension into any A4 Golf and give it better handling and a lower center of gravity.