This can be an expensive adventure. My son's car, which was mine from day one until I turned it over to him with 200,000 plus miles on it, blew black stuff out the back under full throttle too. Oxygen sensor twice, and now injectors.
I would have a compression and leak down test done to check the general health of the valves and cylinders. If the test come out ok, and I would bet they will, why fix what is not broken? These cars are well built, but the overhaul costs are pretty steep. We had a burned exhaust valve and had the head rebuilt after 205,000 miles. The cylinder walls were like new. No ridge at the top of the piston travel, and you can still see the honing on the cylinder walls. No loss of compression in all but the cylinder with the bad valve, and now that one is near new too.
Frequent valve adjustments, synthetic oil changed every 3000 to 5000 miles, air cleaners and that was about it for regular maintenance. I was not frequent enough for the valve adjustments though and ended up letting it go for 30,000 miles or so on the last one, which allowed the valves to get too tight on cylinder number one. Consequently that cylinder's forward most exhaust valve burned. Other than that, this car has been near bullet proof. Never stranded me, ever. Had to push start it a few times, but never did not get me where I was going and has lots of life left in it. Good luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)