The dealer took in my 86, and I have seen a few early 124s in there too. However, I am never taking my car there again since they don't seem to know how to work on an older one. Correct me if I am wrong, but it seems that you actually have to FIND the problem in our old cars rather than just hooking up a scanner and plugging in a new part. I had them try to fix something, then when I went to an independent and told them what they did he just shook his head.
Also, do you think that a dealer, with their 2003 CL600s sitting in there, is going to give your old 10, 15 year old "clunker" its deserved time? I doubt it. Good chance you'll get a hurried job to get it out of the way for the CLK that is waiting.
So I would personally recommend an indy over a dealer in the first place. I'm sure many here will agree with me. Look for one with a tech with some gray hair, chances are he worked on 126s when they were new.
A little note on working on a 126 yourself. It is easier to work on than you might think. If you have any mechanical ability at all, you can do more than spark plugs. With the numerous great write-ups on this site and others (esp. Thomaspin's
www.pindelski.com), you can do about anything. If it's not on his site, search this one and you're likely to find something. I have a mechanical mind, and (before getting my car) have not done more than an oil change and small things like that, and I changed the timing chain in mine successfully amongst many other things. It's a wonderful car for a DIYer.
Good luck and tell us what happens!