Just to add my impressions...
The grins-per-moment I've gotten from my 300E far outweigh the cost of maintenance, which really isn't all that bad. I even grin when I walk up to it in the parking garage at work.
I have been too lazy to research this, but I have a very strong memory of the Car & Driver review of the early 300E. I seem to remember the phrase "best sedan in the world." When I decided to buy one, years later, I was treated to what the C&D editor was talking about.
I would second everything mentioned above to watch for and add the following:
I don't know how many miles is acceptable to you, but if you are looking at an early 124 with around 100K, you will almost certainly be looking at replacing the rear thrust arms (one of the rear suspension links) and perhaps the rear subframe mounts and differential mounts. You can find out more about the thrust arm thing by looking at the "Floating 300E" thread. When mine were done, I had all of the links replaced (not just the thrust arms), along with the subframe mounts and diff mounts for a total cost of $800, parts and labor. Totally different car afterwards. Because of the mileage on my car, shortly thereafter I also had the front and rear shocks replaced - Bilstein HD on all corners. I did the rears myself (easy and a great excuse to clean out the trunk of broken golf tees and dead golfballs and clear out the drain tubes) and had my tech do the fronts (time and tools). Even better handling afterwards. My tech told me that the original shocks would've given up the ghost at around 130K. Based on other German cars I've owned, including an Audi quattro and '76 300D, I think that sounds reasonable.
Check the records to see if the fuel pump relay and/or fuel pump have been replaced (again, on a higher mileage example). Maybe someone with more experience can amplify this, but the relay is bound to fail sometime and failure is a pain in the rear because it always happens at the wrong time. When mine failed (I think they're about $125) my tech told me that the relay might fail if the pump is faulty - drawing too much amperage - so replacing the relay may only be a short-term solution.
300Es, I'm told, may also suffer from "intermittent stall," the causes of which can be diagnosed out the ying-yang and never identified. Mine has never stalled (other than when the fuel pump relay died), but I've driven an 87 and a 90, both of which, oddly enough, the ONE time I drove each of those cars, stalled at stop lights. Started right back up, though.
Finally, I've mentioned this a couple of times on this forum and no one has disputed it, so maybe it's true... I was also told by my guy that MB has changed the injectors from the original design. The original injectors apparently don't work well with the gasoline additives now in fuels that weren't there 15 years ago. This can be the cause of a minor rough idle. Use Techron.
I can't get my outside temperature gauge to work consistently, but that's a minor annoyance. On the positive side, this is also the only Benz I've ever been around with an antenna that has telescoped and retracted fully for an entire year. I read a post about using "White Lightning," a lubricant that I use on the other vehicle mentioned in my signature below. Works like a charm.
I agree with Alex - I love my 124 so much that I'm going to try to pry the 500E with less than 40K languishing in my friend's garage away from the owner. His continued possession of that work of art is a constant friendly tug of war with us, especially since he's currently smitten with a BMW 8-Series.
Sorry for the lack of brevity.
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban