I currently lack the budget and would actually have my car shipped to Europe to have it converted in the UK or Germany if I did, as I have family in France, and the pool of expertise is greater there I believe. Doing it myself would take too long etc.
I recommended buying a German spec. Model, as a parts car for a conversion, because the only things needed for Federal DOT Safety Standards are the lights, windshield, the bumper reinforcements and side doors for the impact beams and maybe the Speedometer and possibly the Airbags. Everything else can remain. Plus it acts as a blueprint. Using your car to Federalize it is a practical option to.
Someone on the East Coast recently took their US-spec RS6 sedan and made it into a “fresh from the Factory” RS6 Avant in terms of “fit and finish”, so anything is possible with the right people and budget when desiring any European-spec Audi.
Please keep in mind, that any engine conversion, on any car, must meet all applicable Federal and State EPA Emission Standards for the year and state for which it was built – Period.
So if you take a gasoline powered 1999 VW /Audi and convert it legally into a TDi it has to meet all emission and emission equipment standards for Diesels sold in the 49 States and / or California for years 1999 or newer. Federal and California EPA regulations allow updating, but not backdating, when switching power plants.
A 1999 Audi A4 is currently, as I recall, on the EPA/ DOT list of legally importable cars along with a list of what is actually required to make them legal in the US.
Britain’s Volkswagen Driver Magazine recently featured a pre face-lifted B5 Passat 1.9TDi converted to an Audi 2.7T V6, with upgrades, that still met all British MOT requirements, which frankly, at the workmanship / compliance level, are much tougher requirements than our own DOT.