Other than the physical size (that it fits), it is important that a replacement fan is the same wattage and speed as the original. Also that the pitch on the blades are the same, as this is what determines the amount of air it moves more than anything.
So I would not guess that a physically similar fan is going to do the same job. Chances are that a fan from any 3 litre M54 engine will be the same, but it is worth checking. If you have an underpowered, or overpowered, fan it will not cool your A/C efficiently (or possible enough).
An underpowered fan, when used as the auxilliary radiator cooling fan, will run for longer and potentially not last long, or not cool the radiator enough.