Hi excellent post, you are in danger of becoming a serious home mechanic, I see Audi are as usual quoting starship part prices and I commend your thinking using the part numbers on eBay thats all I ever do as most of whats classed as aftermarket was used under patent by Audi in the first place, prime example when I was looking at my low pressure EGR cooler valve it was £360+ VAT and is made by VDO/Continental in Germany, the same brand valve with the same VDO part number on eBay £120 shipped, its a no brainer, throttle valve which incorporates the high pressure EGR, Audi over £400 same part eBay £150, if you have the original part numbers its like a kid being locked in a sweetshop, you can try everything.
Steve.