I had a quick look. You have some stray (internal) edges in the models. For instance the turn: see image - the top two circles. You can automatically fix those by installing ThomThom’s solid inspector plugin in SketchUp. You must group your object because it will check if the object is a solid.
On a sidenote; maybe at some time have a look at the amount of detail of the elements. Do you need that much segments in the profile in the middle?
Next; you are getting the ‘object has overlapping uvs’ in Unreal because Unreal needs a lightmap uv channel. The first uv channel is used for the texture of the object (for your metal, wood, brick material etc). A second uv channel is needed for Unreal to store its lightmaps. Unreal can create that second channel for you during import of the fbx (mostly for simple objects) BUT it needs a first uv channel to do so (it re-uses and re-arranges the uv layout of the first channel). So apply a material with a texture in SketchUp and try again. If the error still remains; you must apply a 2nd uv channel yourself in another 3d program like 3dsMax, Maya or Blender.
Finally; making modular pieces seems like a good idea BUT you need some sort of rim / element on the ‘joints’. If you don’t; you will most likely get small differences in shadow & lighting. See more here: Modular Asset Lighting Problem - Rendering - Unreal Engine Forums
