Yup, Valeo is the only listed supplier for the E53 3.0i 140amp alternators.

I assume it's the 6cyl as the 8's all had Bosch water cooled ones AFAIK ($$$).

I've read a lot of people go through a few replacements with remans.

As already posted I would check the battery is still good. Other than that, with the symptoms as you describe, I would be suspecting the alternator myself.

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04 X5 3.0i auto
03 X5 4.6is