The model was finished and the re-topology was underway. This is the high poly wireframe:
I tried to keep it as clean as possible during the high stage so low would be easy, but there were a few issues. Pipes and such were relatively easy, but strange edges provided difficult to remain geometrically accurate without completely re-doing them.
The areas marked in red were pretty messy geometry, with strange artefcats being created during smoothing or accidentally merging verts or something. Either way, it took a lot of work-arounds until I was happy to call it done.
Texturing up next!