Usually a vacuum leak large enough to cause a rough idle will also set the check engine light. Could just be poor maintenance. See if they have the maintenance book or documents supporting recent maintenance. At 60,000 it should have had a major service: spark plugs, air filter, fuel filter, trans fluid change, etc.
Could just be worn out plugs if the service wasn't done, or if wrong plugs were used. Second thought is that this vintage was prone to 2 problems that may cause a concern like this. First is a blown headgasket, second is a bad engine wiring harness.
I'd pay to have it inspected by an MB dealer or trusted MB shop.
If you can, get in the car and just turn the key to "on" and make sure the check engine light lights up, to make sure someone didn't just pull the bulb out.
I've never heard of a temp indicator complaint exactly like this. My guess would be the display itself, which is replaceable seperately.