I couldn't agree more with Scott and Stick's comments regarding the earlier diesel engines as used in the W115's and W123's.
Injectors and glow plugs don't last forever and the quality of the diesel used also plays a major role in the longevity of said components. Neglect to properly maintain these older diesel engines usually is responsible for the myth that these engines are hard to start and temperamental. Neglect (i.e. failure to adhere to regular oil change intervals) also lead to premature wear of the cylinder/rings and hence lower compression and increased blow by which contributes to the difficulty in starting the engine.
The more powerful starter is used to overcome the higher compression ratio of diesel engines and not because they are inherently more difficult to start after a few years.
Lastly, W114's don't have diesel engines, only 6 cylinder gasoline engines. W115's have 4 cylinder gasoline and 4 or 5 cylinder diesel engines.