Consider repairing your water pump.
Also search all part numbers, not just BMW. It may be an FAI pump, part number WP6436.
I found an S50 water pump for £30. The guy was selling stuff that was from a bankrupt warehouse. All he knew was the part number on the box. He had no idea what it fitted, nor how rare it was.
I've kept the old one - it just needs a new seal kit and bearing when the time comes.