You've stumbled across my Blogger post. I have documented some of my findings whilst owning my Audi A4 B7 2008 Avant to not only help other people, but also so I can refer back to them ;c)
Workshop/Repair/Service manuals - You can actually get the entire collection of full workshop downloadable PDF manuals directly from Audi for 34 euros (price @ 2017). Go to https://erwin.audi.com/erwin and purchase 24 hours access to the VW/Audi/Seat workshop manual, once you download the manuals they are yours indefinitely.
VAGCOM Cables - If your looking to hook up the OBD II connector and use Ross-tech software on your laptop you'll need not just any cable off ebay but at the B7 is dual K-line plus CAN it's going to have to be the more expensive HEX-USB+CAN lead. I just paid $80AUD (current 2017) but that included the Rosstech software.
Programming Key FOBS - I've read that you can program your own keyfobs with the VAG COM cable and software which is big saving Audi in Perth charge over $250AUD (current 2017) for this service.
Coil Packs - I just had an incident where I put 20L of e10 at the petrol station and 30 meters down the engine light come on and the engine was spluttering and misfiring. I drove to the next petrol station and put 20L of premium fuel in thinking it was fuel related. I limped him and put the code reader on and found cylinder "1" mis-fire. At this point I thought it was important to drain the petrol completely and fill with a fresh tank (see drain tank procedure). After changing the petrol I still had the problem. I decided to use the isolation method, but as the coil pack harness is ridged it wasn't possible to isolate coils instead I used the injector plugs I found that disconnecting injector 1 made no difference to the idle, I then swapped coil 1 into cylinder 2 and visa versa and then did the isolation test again and found the problem followed. I cleared the codes and went for a drive but dispite the really badly running engine and flashing engine light no codes were produced (i was looking for misfire on cylinder 2 to appear). I was pretty happy that is was the coil pack and luckily my local parts shop (super cheap autos) listed them in stock for $73AUD (in year 2017), when I went down they said they don't keep them in stock but they will be overnighted and ready the next day. When the coil pack arrived it was actually a Bosch unit which fitted in nicely to the car and once I started the car it was running good again.
Drain Petrol Tank - This can be done in less than 15mins. Using 2 spanners unscrew the petrol line on the fuel rail in the engine bay and place it in an empty 2L coke bottle. Locate the fuse box (it's in between the drivers down and the dashboard, flick off the flap with flat blade screw driver), find and remove the fuel pump fuse. All you need to do is put 12V onto the fuel pump fuse terminal, for safety (and save dam$$age to the fuel pump) you should make sure to put the fuse inline with the wire. This method is actually the approved method in the workshop manual with the exception they use a "special trigger like tool" instead of a standard wire between battery but the principle is exactly the same.
LED Bulbs - One of the earlier things I did was replace all the interior/number plate/park globes for LED globes. The first thing you should know is unlike a standard car, the globes are all managed by the comfort control system which manages the globes and keeps an eye on them to see if they are blown. Because of this you can't just stick standard LED globes in because they don't drawn enough current and the computer will keep telling you they are blown. When you are in ebay add the keyword "CANBUS" to your search, they are the same price, but they have built-in resistors and heatsinks to make them draw the same current/amps as a standard filament globe making them CANBUS friendly (eg you won't get errors).
Footwell lights - If the guy who bought your Audi from the Dealership didn't pay for the optional extra "interior light" package then you are probably missing a few lights from your interior such as footwell lights this guide here will show you how to retro fit http://www.audi-sport.net/xf/threads/my-diy-guide-for-retro-fitting-foot-well-lights.58215/
Door lights - You need VAGcom to turn on the courtesy lights. There is a guide here about retro fitting the door lights http://www.audi-sport.net/xf/threads/retro-fit-warning-puddle-lights-a4-b7-salon-how-to.138473/
