I have one myself...first, you need to change your oil every 3,000 miles faithfully. If your car is in as nice a condition as you indicate, the answer to "how long will it last?" is .... almost forever.

I'd get a copy of the shop manual by Haynes for the W123 diesels. This SHOULD spell out what maintenance steps you or your mechanic need to take at what interval. I'd also make sure I had the owners manual that came with the car.

To bring you up to speed on Mercedes, I'd suggest joining the Mercedes Benz Club of America, at most for their magazine The Star. As important as the editorial content are the ads for parts and service. A paperback history "Mercedes Benz Buyers Guide" by Frank Barrett is available at all large bookstores. It is most valuable with hints as to what to look for/look out for in cars like yours.

Value is a paradox: you can buy the most perfect W123 car in the country for around $5,000...but you will have an almost perfect and indestructable machine, easy and cheap to keep up, and perhaps the last truly "permanent" car to be built.

There are a lot of Mercedes people in Maine, including one guy who shows up at far-flung events in a '70s sedan which would be ordinary except for its condition. He always drives off with a prize.

BEST of luck.