My boy is selling his 12k HID bulbs, just wondering if those will work with the stock HID projectors on the 01 A4.

As long as they're the same bulb style as you have already (ie, H7, H3, etc) then they should fit in there with no problem. Thing about it is, 12k HID is not very efficient for lighting the road well. It looks cool, but your visibility may suffer. If I were you, I'd buy a kit with a color temperature of 4300k to 6000k - they're all very white light on the road, and look great as well.