Your 1999 Z3 2.5 M52 has a plastic water pipe under the manifold, the Heater Inlet Pipe, 11531705210. This is apparently the only plastic pipe on that engine, according to the view in RealOEM which also has a photo that can be enlarged:
Going back in RealOEM to the first 2.5L 6-cyl M52 engine in 01/1998, the same pipe is shown as plastic.
Going back in RealOEM to the first 1.9L 4-cyl M44 engine in 07/1995, a similar pipe is shown as plastic except instead of being under Cooling System, it is under Engine Housing.
I have not heard that this pipe was ever metal as an OEM part. Such a pipe in metal, with all its mounts and fittings, would be very expensive to manufacture.
To get an idea of what replacing your heater inlet pipe might be like, here is a video about rebuilding the cooling system on a 4-cyl M44. That pipe shows up in Msg # 5.