Jurgen, you can find nice ones for around $3500 if you're willing to fly out to get it. If you look long enough you might find one locally. I just picked up a rust free 91 E30 with a bad engine (one broken camshaft..very replaceable) for $750, but a replacement upgrade drivetrain was found for less than the car cost me. I'll have less money in this thing than it will cost me to rebuild my 915 tranny!
Here's one in Texas...maybe rust free:
http://www.e30tech.com/forum/viewtopic.php?t=38384
Keep an eye on the E30tech.com ads, also check out r3vlimited.com. I've got some others that I drop in on, but most are frequented by the same people, so the cars usually appear in them all at the same time.
The E30 is very easy to work on. It's a light car and more fun to drive than you would think coming from a 911 background. I have an e30 325e that I'm parting out. It's a runner, but too rusted for most people...but even it (eta engine with a Jim Conforti chip) was a blast to tear up my back road test loop.