Yes, the Sports diff! Unfortuantely very rare, but well worth the money!
When I bought my B8 (not an S4 unfortunately) must haves were B&O and Nav, but Nav can be re-trofitted in a day for not thaat much money.
Depending on the generation of the car (facelift or not) most likely you won't have bluetooth audio via A2DP. There is MMI 2G, MMI 3G both with high/low and MMI 3G Plus (facelift only), I converted from MMI 2G High to MMI 3G Plus high.
B&O is pretty decent, the subwoofer is weak but can be upgraded easily and then it becomes quite good.
AMI (Audi Media Interface) is also nice, but can be retroffited. You can also potentially get Bluetooth through it.
Other driving aids are quite nice - Adaptive Cruise and Brake Guard are old tech and not the smoothest in the world, but I still like them on my B8.
