i did my timing chains about 2 years ago and fitted a cheapish pump around £60-70 and it failed, (immediately, but so slight i hadnt found the leak for a good few weeks after finding others!) it started leaking, but was hard to determine, untill it was leaving stains, and i had a mirror on a telescopic pole. but when i checked it. the actual casting under the bearing was weeping through a 'crack' (from manufacture, not bad fitting) however, with another similar priced one its been ok for a year and half.
i would go to a bmw forum, put in 'm62tub44 water pump OEM' and go from there.
one last tip, the o rings on the long pipes front to rear of engine are easily mis-aligned and leak water. its very easy to pull the pipes half out (the rear end) as you remove the pump, and then not seal very well and then a leak from that area (front or rear) of engine can result. if you somehow brace the pipes with something when pulling water pump carefully forward, this should reduce chances etc, and order a spare water pump gasket. they should come with one, but are a bit fiddly to offer up and stay in situ, hence there's a chance, that with a bad bolt up and a small leak, that it may well need to be taken off and put back on, and the gasket could tear, (they are held in place with some RTF sealant) resulting in another wait for another gasket. they are only about £3
i know you are having it done for you, but i watched a few youtubes, on that waterpump.
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