I've been burning a ton of cash trying to fix a rough idle problem on the 1994 SL500. I've replaced everything from enginer mounts to throttle bodies to distributor caps and ignition wires. The dealer has checked the compression, intake manifold seals, injector spray pattern and for vacuum leaks. They even swapped out the air flow sensor to see if that could be a problem. After all of this... my engine still idles somewhat rough. It gets worse when the outside air is cold and damp. It can get bad enough, that I feel like I'm riding a Harley. I'm at wits end and close to an empty wallet. Can anyone give me any advice on what steps to take next. (I love my car... so dumping it isn't an option... yet.)

I would hook up the engine to an ignitionscope.
That should tell you something, i.e. the cylinder(s) doing more & less work.

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2007 C 230 Sport.

I've asked my dealer to do this numerous times, but their scope is broken. That really surprised me. Is this a commonly used piece of test equipment these days?

To diagnose problems, as described by you, yes.
Btw. you can go to any decent garage for this.