Need help with the creation and importing of 3d models.

This is my first project where I am actually making my 3d models from scratch. I'm not sure how it should be done, here are all of my concerns.

• What program should I use, I already know how to use SketchUp, but if there is a better program, I don't mind learning it because it will help me in the long run.

• I've looked in the documentation about what 3d file formats are accepted, but is there one that is better than the rest?

• When doing animation, do I make each separate moving part its own 3d model and combine them in unity. Or is there some way of separating all of the different moving parts in the 3d drawing program I'm using while keeping it all in the same model, or do I do the animation in the 3d drawing program?

• Whenever I've downloaded 3d models from the unity store, the 3d model is comprised of more than one file, do I need to worry about this?

• Is there anything else I should know?