Welcome to the 124.133 fan club! Great car, you'll like it after you get it all tuned up. DO NOT convert the AC to R-134a if it is still an R-12 system. If already converted, oh well. You shouldn't be wiping black off the dash. Perhaps a previous owner tried dying it, and that is wiping off? Best thing to use is Formula 303, and get the Mercedes factory windshield sun shade and use it EVERY time you park the car.
The steering wheel is horrible on these cars. Get a 1990-1991 leather steering wheel from a 300E or 300D, it's a 100% bolt-on and uses your original airbag. 1992-up needs a different (expensive) airbag. The 90-91 wheels are available from salvage yards for ~$75, try PGAuto.com or SilverStar in CA. I installed a 1998 W210 wheel & airbag, which ain't cheap, but feels awesome. Here's photos:
http://www.meimann.com/images/mercedes/W124_stuff/
(note that my "old" wheel was the 90-91 leather type.)
Check out the other stuff on my website too, I've done lots of mods...
