The belt in my car is a V belt.

I am concerned about the mounting bracket - I do not know if the alternator that fits in a diesel W123 will fit in mine (which is 200, not 200e - a model with a carburetor).

As for power - the current alternator provides 660W of electrical power and probably loads the engine at ~800W. If the bigger one takes 1.5kW from the engine I most likely won't notice (I probably will notice when I install an AC in my car).

200D is 40-44kW engine, mine is 80kW, so much more power.