My brother had a 1985 190e 2.3 and it was a great little car. He drove the hell out of it, and didn't maintain it at all, and it was still very reliable.
He drove it from sonoma California, to Los Angeles, on one tank of gas with hardly a drop of oil in the engine, and the car was fine still. It took 5 quarts of oil when I finally met up with him just a few miles north of los angeles to buy him gas and put in oil.
The car lasted for 3 years after that incident, and only had minor repairs, alternator, radiator, etc. Never anything major.
The 2.3 is a very capable car, great gas mileage, and will easily do 120 mph on the highway if wanted.
the 2.6 is also very nice, smooth and gets great gas mileage.
If you ask me you can't go wrong with either car, I'd go for the newest one you can afford.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.