The A4 (and the Passat) use a longitudinal 1.8T so you would need a K04-015. It bolts up to the stock exhaust mani, but you still do need software and a few other things to make it run right. It really depends on the file. Some files are written for larger injectors, a full exhaust, FMIC, etc.

You car is a 2000 so im guessing an ATW engine code? Not sure about A4s to much. The K04 is a good little upgrade, its not going to do anything crazy for you though.

Is your car chipped already? If not, I would definitely look into that before a turbo upgrade. If you go over to Eurotech and get a GIAC chip, that will give you 40+ hp and probably 50ish+ lbs, all for ~$500. Get a chip, exhaust, and FMIC, and if you still want more power after that you can go to a K04. GIAC offers an upgrade deal so if you already had a chip with them, you can upgrade to the K04 file for not much more.

You can buy the turbo here: http://www.kineticmotorsport.com/pro...15-37-141.html

and at a minimum you need the install kit with it:
http://www.kineticmotorsport.com/pro...t-509-141.html

Sorry for the broad info, i can help more if you have some specific questions about it.