Welcome Chris
I will let the D3 Owners on here comment on their 3.0 tdi's etc but your mechanic in my opinion needs to answer the following - what specific evidence does he have for too many things going wrong on these cars? what examples?
Has he ever owned one and what other cars is he comparing that against and is it therefore a fair comparison? What fuel consumption does he expect for a vehicle of the size/type of a D3 A8 and why does he feel its worse than competitor brands? Is a 3.0 tdi in an Aluminium A8 worse fuel wise than a 2.0 tdi in a heavier steel A6 for example?
What things is he saying are expensive to replace and what particular parts does he think are likely to fail that are more costly?
Lastly, perhaps the reason you havnt landed one yet is because they are all so good they are getting snapped up quickly by others?
I'm just playing devil's advocate here but have a browse around each section of the D3 forums and you will build up an idea of what issues crop up and how easily or otherwise they are solved. A8 Parts (Prestige Motor Services); the Forum sponsors, exist to provider us with a cheaper-than-Audi spares and replacement parts service for many items.
Again welcome and I hope that you find the right '8 for you and that you can see past the "generalized comments" that many people seem to make in these situations without full awareness or knowledge  Good luck!
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Confidence is the feeling you have before you know better . . . . 
Audi A8 D2 3.7 Quattro Sport 1999 FL - Melange
2nd Place Winner - A8-Freunde Annual German Meet 2012 at Edersee
20" x 9" Bentley GTC Wheels with 385mm W12 Front Discs & Bentley Caliper Covers, Front Lowered on Coilovers, Xenons, Combined DRLs and LED Foglights to modified OEM front fog lamp position, Alcantara/Leather, Full Black Alcantara headlining + pillars and blind shelf, Bespoke Flat Bottomed Sports Steering Wheel, Dark Myrtle Wood, Autodim Mirrors, Bose, Blinds, Digital TV, Ski Hatch, Modified rear exhaust section for extra V8 Burble 
Audi S8 D2 Final Edition 2002, Avus Silver with Red Leather Interior
Tiptronic with Sport mode button, Sony rear entertainment package, Bose sound system, Audi Navigation Plus, 20 inch Final Edition alloys, Full Cat Back Stainless Milltek exhaust, giving a great V8 burble
Audi A8L D2 4.2 Petrol with LPG - Ebony Black
Audi A8L D4 FL 4.0T Petrol twin turbo - Havana Black
Audi A2 1.6 FSi Sport - Dolphin Grey with Votex body kit and Red Leather Interior
and some other A8's A8L's and Mercs
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