HI there, sorry for my very slow reply to your message, I try hard to help out my fellow Audi owners.
Why go for a 170 turbo, there are some very good options around to take it higher that are better than the standard turbo. The only time I would do this is if I wanted to keep the cost down and found a suitable 170 turbo from a breakers yard.
In the A6 the engine is longitudinally mounted, so any turbo setup for this mount should fit ok. Don't go with any of the transversely mounted engine turbos, the pipes and setup will not work.
I can't give a specific part number, there are too many variables, and even a good garage would want to see the car and check the serial number from the old turbo, just in case the head has been swapped out or there is something on it from a previous or later production run. I think you'll be safe with any 170 turbo from a similar year car as most of the oil/air and mounts are the same IIRC.
If I was to choose a turbo for the 2.0 for a decent remap and sensible power figure of around 225bhp, I'd go with a Garrett GTB1756BK or a 2260VK if you are happy with a little low end lag and more top end pull.