1 - Answer: Not sure about the exacts of it but, if you do have them as d2s, most likely they are ready for d2s or Xenon bulbs. Sometimes, you might have to get an adapter of some sort (i.e. H7 to d2s etc..) I had Xenons that came with the car hence, I cannot help you much there since I don't know what the Halogen type look like. Perhaps your first place to dig I would imagine is to find if you can place D2S bulbs in those existing projectors.. If you do not have Projectors in the headlight, you have to go with D2R type bulb. In either case, you need the Ballasts, Ignitors and wiring Harness... (Hella's kit for A6 or Audi is nice because it has the Ignitors built in ~ also, they are the OEM for Audi A6).

2 - Answer: If you do not have Projectors already, why don't you go with D2R bulbs? That's what they are made for. If you are dead set on projectors, you might have to modify Headlight housing (may involve taking the entire headlight out, cutting / sheilding etc. as well as mounting the projectors...) but since you mentioned that your car is A6 4.2, I believe they have the same headlights as the 2.7T (mine) type...(without xenon in your case)... if that's the case, there is a section in the bottom of the headlight housing (already marked) that you can cut out to mount the Ballasts into... as for wiring, you might want to get the ballasts with the PROPER Wiring Otherwise, you will risk buring up the whole thing....

A word about voltages: Xenon bulbs do not run on 12vdc... they require High Voltage...that's why the ballast is there.. Since Xenons area ARC based lamps, they have a Kickstart stage and running stage… In the Kickstart stage, the voltage may rise to ~400v and running I believe is ~85v…. Either case, lethal and shocking voltages are present. Plus, if you miswire them, you run the risk of fire. Proper mounting of bulbs is also important due to the High temperatures the bulb generates… So, make sure you get proper adapter if you need it.

So, check if you can put D2R bulbs in the exisiting projectors / housings… then if that is not an option, try getting A6 projectors… and D2S bulbs. Either case, you need Ballasts and wiring. If you have the Halogen (without projectors), then you also need adapters… (stay away from ReBased bulbs…) they work but not recommended / reliable...

Part numbers??? Contact local Audi Dealer and check with their parts dept.

3 - Answer: To be honest with you, stay away from Bi-Xenons… Finally it's your choice but I personally don’t think they are worth it. Here is the reason why: I don't know if you have noticed but, A6 Projectors (lowbeam only) are some of the finest projectors out there…. If not, The Best. Now that can tickle someone reading this… but either case,

They have excellent output, very nice color (cut off color) and spread light well.

I don't know how much you know about this topic altogether…so, I ran off on a tangent..

The problem is that when you get Bi-Xenons, the cut off shield is very hard to modify to get the color output you want…that's the main reason why I do not like them. On the other hand, how many times do you use the HI-Beam?

If you drive around the country side and not much traffic, then it might be wise to invest in Bi-Xenons… other wise, stay away from them. Plus, you need to modify the wiring to disable the OEM Hi-Beam setup…. Too much hastle if you ask me…

If you want, you can email me and I will try to help you with any questions….