If you see my signature you will notice my car. I got it with 450,000 miles on it. With that many miles you can expect a lot of suspension parts are out. I replaced ball joints, tie rods, etc, all the steering components except the steering box which is worn out. That cost me about 1000.
I replaced the radiator, hoses, and belts, that was about 400.
I got new wheels $500. Got more new wheels 1k, now I need tires $600.
I got new half axles from napa (dont get mercedes parts from any place but mercedes dealers, places like this (mercedesshop.com) because the quality sucks. But I did those and replaced a wheel hub because with the new wheels I got a short lug nut and it stripped out on of the holes. That was 600 including labor.
I got the valves adjusted, injection pump timing adjusted, a new compressor, drier, etc for the AC, new push buttons, etc. That was 1100.
And since then, which was last summer when I did all that, the only money I have spent on my car is for EURO lights $300. Then 2 Oil changes, 1 tranny change, diff change, coolant change, flushed the brake fluid, got some other calipers because mine were leaking, and thats about it really.
I think if you keep them maintained they will go forever. Sometimes rubber parts wear out so you have to deal with those when you come to them. It may seem like a lot from what I have done to my car, but my car has a lot of miles on it, so a car with much less you wouldnt have to do what I have done. Just keep the oil changed, thats the most important and keep the fuel filters changed because bad deisel screws stuff up.
Id say that yearly you could look to spend $200. But if something catastropic goes wrong its usually about 1k dollars.
Id say when you are looking for a car, the most important thing would be making sure they have records of maintence, make sure that the car wasn't beaten. Check to see if the oil is low, if its low, its probably always low. Ask if they check their oil, etc.
I think that 300Ds are very reliable, and they are safe. My friend bought a brand new car, and it breaks all the time. My car hardly ever brakes, and when it does, if its not cheap, you have to deal with it.
No Benzes in the stable at the moment, but itching for one again.
Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 300E Amg Hammer 88k mi sold
1992 500E 156k mi sold