Here are some sites that will doubtless be of interest -- I don't know
how much any of them go into actual cars for sale, but some of them
tell you in great depth what to look out for and how to maintain the
thing once you've got it.

http://www.washdcmail.com/02_page/02_main.html

http://www.bmwcca.org

I don't own one, but  a friend who has an ever-changing collection of
the things loaned me one for a few months because his driveway runneth
over, and even though that example was a commute beater / "before"
stage of a rolling resto, it was easy to see why he likes them so
much.  Its objective numbers are pretty pedestrian by today's
standards, but it's a very honest chassis -- you soon get to feel that
you know what any control input will do, and then that's what happens.
  And unlike some sports cars that make me feel like I need a
shoehorn to get in and a periscope to see out, it has something
approaching a sedan cabin, and great sight lines.

Watch out for hidden rust, don't overtemp it, if you luck into one
with air conditioning don't expect that feature to accomplish quite
what it would on a modern car, and budget for maintenance like it's
the 30-year-old car that it is, and you'll probably be pretty happy.

Best of luck,
--Joe