I'm not the wheel expert here so hopefully someone can correct me, but as I recall, the Audi center bore is smaller than the MB center bore, so you may have to find someone who can re-drill, and I'm not sure how much effort that would take.
[edit: Audi TT center bore 57.1, Mercedes center bore 66.5]
This center bore issue may be why it's easier to see Audis with MB wheels than the other way around.
The offsets don't sound unworkable, but again, this is not my area of expertise. If possible, you may just want to see if the seller will let you test fit the wheels.
You can give Sander at The Wheel Supply in Walnut, CA and get his oppinion as he's quite familiar with non-typical fitments for our cars.