Assuming you are refering to the M111 four cylinder, most agree they are very reliable although a little coarse. The M111 will not be as smooth or quiet as your 300CE (assuming it has the M103) but should prove more reliable and should also be more reliable than an M104 (eg. 300CE-24, early C280, E280 or E320). Unless power is of great importance to you, I would be reluctant to dispose of an M103 powered car (assuming your 300CE is) for one with an M104.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.