as mentioned above fella the center bore, PCD and bolt thickness are key.
The center bore on Audi's is 57.1 where merc have a center bore of 66.6 - that said Audi have started using the same center bore as mercedes but thats on modles that were manufactured in Q2 of 2010.
If you do go for wheels that have a CB of 57.1 you'll need to get an enginering firm to mill out the CB to 66.6.
Some Merc's have a bolt thickness of M12 where all Audi's have a bolt thickness of M14 so worth checking that too for a snug fit.