Pump is now pulled. It has a metal impeller and is a BMW part. I have a feeling it's the original pump, so it makes sense that it blew up around now at 178,000km.
Here's how broken it is:
Next up is to replace the thermostat and housing while I'm there. I have an aluminium URO housing, but I've heard that people think they're crap. Any opinions?
Here's my "while I'm in there" parts list:
Mechanical tensioner and both tensioning pulleys
Water pump pulley