The fuel consumption will not become the biggest or most expensive problem. Real dramatic is the spare parts situation. Although it's looking like a pimped Audi 100 or 200, very many parts -especially the exclusive oily bits- were V8-unique. Which means
- not many made
- sometimes handmade, because meant for an exclusive premium cars
- with the appropriate prices for an exclusive premium car
- hard to get, because due the few cars and the high prices not many Audi-garages have stored them
- hard to get, because compared with Mercedes Benz and Porsche Audi was never engaged in the spare parts supply for older and rarer models
These warning you can read in any buyers guide article for the V8 and the freaks
http://www.audiv8.com/ always state that explicity. This is indeed the reason, why the prices for the V8 are quite low and why often unexperienced newbie-fans are selling their V8's very soon again.
As walter is a long experienced Audi-nut, I'm sure, that he's aware all that topics.