I went back and re-downloaded some of these models as 2017 .skp as opposed to the .dae and after opening and re-saving in SketchUp as .fbx I can now make them work without the weird texturing issue. I still have no idea what it was though...
...every single model that I'd gotten from 3D Warehouse (BSG) as .dae was the same issue and no amount of polygon poking and material manipulation could sort it out. So in conclusion, if you see this issue then my advice is to download as .skp and use SketchUp to save out the file as .fbx and convert it to .obj for Terragen in your preferred 3D app (I use C4D) and they import fine. They also now and render in C4D properly too, so...