Take a look at this page I started a long time ago but never finished.
http://raptusregaliter.gig4free.com/W123/index.html
The important thing is to get the ignition out and figure out what the problem is before it fails completely.
Your problem sounds very similar to what I was having with my car.
Your mileage will varry with a donor from a yard. The main problem you'll run into is, its tough to get these things out without the key. If the yard has the key, odds are they will charge you an arm and a leg for the assembly (my local yard wants $120 for it. New is $169.00 from Phil here at Mercedes shop.