Barrier hoses are typically required for conversion to R-134, but work very well with R-12 as well. There is an actual membrane in the hose which keeps the molecules from migrating through the hose. Barrier hose is about all you can find today.

My car had the York compressor so it was simply an exercise in bolting on the adapter and then attaching the compressor to it. I have limited experience with the A6 but it can't be all that hard if you have the right tools.

It's hard to compare the before and after (the before was in the Fall when it's much cooler here) but with the Sanden compressor and the parallel condenser I have not had any issues with cooling. I have had issues with the ACC servo adding heat when it's not needed but that's another issue. The barrier hoses should all but elminate any need to top off the freon.

I recommend R-12 over R-134. It's the same price, if not cheaper, and works very well.