For the mod I'm working on I used CraftStudio to make the models. CraftStudio works but I find it awkward to use and the viewport too small.  Today I found out I can make models in Sketchup (which I'm familiar and comfortable with), export them as a DAE file, import it into blender then export it as an obj. Making textures for models made in CS is fairly straightforward, there's a panel where you move the blocks' wireframes to set what part of the texture map that that block gets its textures from. But for something made in a proper modeller there'd be triangles and perhaps spheres (like ExUtil's impossible objects). How would those be textured?

Search for tutorials on how to texture models in Blender. Exporting as an .OBJ requires you to have a .MTL file as well, containing all of your materials / meshes. For a triangular model, it's pretty simple in blender, basically you go into UV editing mode, export your UV template, create your textures, create a new mesh, import your texture that you made, and map them to your model. After that, export it and you should be good. I am no expert in Blender or Sketchup by any means - go watch a tutorial. (BTW you can also use pre-made textures and map them to your model, if you have them)

That sounds complicated, I don't even know how to make models in blender but I'll look for tutorials.