I had the 2.6 V6 in a 1992 Coupe and its engine was smooth as butter!! Only had 150bhp though put it pulled well in the mid range. Sold it last year and the body was immaculate, no rust whatsoever and only some minor stone chips, and the door closed with a more solid 'clunk' than my current 2000 330Ci!!
My parents had a few Audi 100s, some 5-cyl and one 2.8 6-cyl. I really really love the 5-cyl engines. They run very smooth but step on it and they roar . The 2.8 one was a lot faster of course, but at lower revs it wasn't that much faster than the 5-cyl ones. And the V6 was too quiet for my tastes, although when it made some noise (above 5500rpm) it was a great one
The best thing about that car is the HUGE amount of space you had to work on things under the hood! I had the V6 (2.8) and I could almost sit inside the engine bay to do most things, it was great! Plus it is probably the easiest car on planet earth to do an oil change on.