Hello Michael,
Any 30 year old car which has only covered a genuine and verifiable 4,000 miles from new will certainly have a high interest ‘value’ irrespective of the make or model, and an A6 Quattro example will certainly enhance that appeal.
Having said that, the way in which it’s been stored over those years, will obviously influence its condition and value.
I would say it would be impossible to sensibly value the car, and the only meaningful way to find its true value would be to enter it into the hands of a respected classic car auctioneer. Even then, the bids will be variable dependent on the sale/sales.
I recall entering my 9K from new Morris 1000 into a classic car auction decades ago, and the auctioneer’s quote was ‘ it will sell for whatever someone will write a cheque out for on the day, and that will be different to what someone will write out tomorrow’. Not very helpful, but true.!
Summary:- Get the car presented to best effect, and enter it into a classic car auction in Germany.
Perhaps you can let us know how you get on.
Kind regards,
Gareth.