I do hope the the trans problem is something external and electrical
'cuz if you need a rebuilt for one of those larger quattro transaxles, it's gunna be at least $6000., not $1500., according to what I've read over on audiworld.com (which would be a good site for you to visit for information and inspiration). Audi motors have had high points and low points, but the longevity of the V8 motor seems to be excellent, like that of the V6 which was cloned from it, as long as timing belts are given their due attention -- I've got to change it on my daughter's '93 90S (for sale) soon, since she's past 181,000 miles now.
A '73 100S??? They almost all cracked their heads before 100,000 miles, and that motor was Daimler-Benz design!