I love my '96 S500 coupe, but would have to honestly tell you that if maintenance and operating costs are a significant factor, I'd look for something else. These are pretty complicated cars and the parts are expensive. (call your local MB dealer and ask for a price to change out the A/C evaporator) :-) With rare exception, you won't be buying your parts at the local Autozone.
Having said that, the engineering is to be admired and the execution is wonderful. Drive the car on the highway at about 70 with the windows rolled up and you'll see what I mean. Like sitting in a bank vault.
No way anyone can assess whether or not you can do your own maintenance without a lot more information. I maintain mine but its not job for the faint of heart.
One repair, considered inexpensive and easy on some cars, can break your wallet for good. If you've searched the site, you'll find the the W140's are pretty controversial... some love 'em, some don't.
If you must have a W140 Coupe, I'd look for a '96 or newer. They were significantly redesigned in '96 even though they look the same.
Hope this helps.

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KenP

1996 S500 W140 Coupe