*About UV exporting
Blender has a large UV toolset built in, you dont have to use blender to model, but I highly recommend you use it for modelling. There is a great community around it, and I just taught an IRL friend how to do the basics (what you want to do) in about an hour.
[if using sketchup] Go to file-export-3d model, then export a .DAE from sketchup
[if using sketchup] Import the DAE into blender using file-import-collada(.dae)
unwrap the mesh in blender (if you don't know how, use this nice tutorial)
Export the UV layout from blender
texture in your software of choice (i dont recommend using MSpaint, something like paint.net would better suit your needs)
I'll answer you in more depth answers to your other questions and give you more resources for learning blender when I get home in a couple hours.