developing a CAD tool

I can't see why not. Unity allow you to control meshes arbitrarily. Of course you need to put in some effort to create all the controls and algorithms necessary. If you're targeting web builds you need to know you can't save local files (unless they changed this, I haven't used Unity for a while now). You can however communicate with the webpage and possibly use web controls to save your files. Also you can communicate with some file server. If you're targeting standalone programs then this won't be an issue.

You could do that.

It really depends on the investment you are going to put into it.

The unity editor is wonderful for creating a 3D world.