I made this conversion quite some months back, and thankfully I finally got around to it...
I wanted to try and do something with different types of metals. It seemed reasonable to me that highly modified members of the machine cult would consider certain cheaper metals suitable for use in expendable throw-away bits of kit (e.g. the power plant on it's back), and others, such as the more integral parts of their systems to be made to last in a more permanent capacity, and more exotic, rare metals might be used (e.g. the face plate and re-engineered weapon, which is based around a xenos design, incorporating some bits of xenos and imperial tech).
