I finally figured out (with the aid of the internet) how to fix the issue that was preventing me from importing the running-gear file, so I could put together this mock-up assembly of all the components.
For 3d printing purposes, the colour is really irrelevant. I thought I'd give it some though, because why not? In the process, I discovered that the turret and sub-turret files really need a bit of refinement. They were my first attempts in Blender, and it does rather show.
Now it's on to 3d printing the thing. I don't have local access to a printer, unfortunately. Even a relatively low-resolution printer would be useful for testing purposes, since the alternative would be to get Shapeways or one of those companies to do it, which would likely end up costing me an arm and a leg before I get a successful product.