I would say from the top of the head its not that hard..
there are 2 bolts that hold the altenator on.
you can see 1 from the top side of the motor and 1 you have to just feel your way around underneath. Or lift the car up and then you can get to it.
The one on the top is also the one that adjusts the tightness of the belt. Loosen the back side bolt then adjust the altenator with the front side so that you can take off the belt.
Then its also a matter of taking off the wires from the wiring harness and taking out the last bolt.
I think on E34.de there may be pictures.
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