I was working on my car one day and ended up not needing a part I had picked up at the Audi Dealer, So instead of showering and changing, I walked in the nice showroom and across to the parts desk in greased up jeans and shirt, I can tell you no salesman bothered me that day. They probably got annoyed that I showed that people actually "have" to work on their cars. But anyways, I seriously doubt MB changed your Timing Belt, "may" have done the oil, but doubt air filter, just get your hands dirty and take a look. If the belt has a big line down the middle its probably not new, check the edges for cracks, not a DIY if no knowledge. You can buy a OE drop-in air filter for cheap, search web, can be a pain to install at first, also ensure to run fullsynthetic oil, much debate on what brand and wt, but surely Fully Synthetic, also, it may be expensive but stick with mid-grade if you have a stock ECU, and run a can of Sea-Foam through the gas tank ($7 at AutoZone) In fact, that would be my starting point, go to AutoZone, check the battery, they may have a Vag tool (ECU check), run some SeaFoam in it, fill up with some mid-grade, check the oil level, if low fill (i hope they used synthetic), if its dark black in color, change and use new oil filter (order online either Mahle or Mann, or Bosch at local parts store). You could also get some electrical parts cleaner (MAFS cleaner) from the store and clean your MAFS, little complicated at first, but do some searching for how to. Good luck.

'01 A6 2.7T Quattro
Performance: APR Stage 1 ~ APR Cat-back ~ EVO V-flow CAI ~ Vogtland sport suspension (Koni's) ~ H-sport sways front/back ~ R1 Concepts Driled/Slotted Front/Back ~ Mintex Redbox ~ 18" RS4(Achtuning reps)w/ Bridgestone Potenza RE760s 235/40ZR18 (summers) ~ Tip00032
Reliability: Samco TBB ~ Samco Upper Boost Hoses ~ Forge DV ~ Castrol Euro Syntec w/ MANN ~ 93oct ~ Timing belt/Valve covers/and plenty more (trust me).
Eye candy: RS6 trunk lip spoiler ~ FK grille ~ B-Raudi6 Deutsch front tag ~ Smoked side markers ~ 30% Tint ~ A6 All-season rubber mats ~ LEDs-- Red globe/driver's feet; Hyperwhite License plate/Inner trunk/Vanity mirrors ~ Smoked plate cover/Quattro logo trim ~ A6 debadged- 2.7T Quattro relocated to drivers side.
Future plans: Piggies/Stage 2+, GIAC Tipchip