To show the design, build scale rings and negociate with a customer, something like powerpoint or Impress(Open Office.org (free)) is good. I use that kind of stuff to discuss new products I wish to have made. I even made a scale model of a set of ringwork I was unsure of how it would look on the cue. That turned out well. With Impress, you can use images of a material as the fill pattern and get a pretty real impression when needed.
If you need or want basic CAD, Dassault Systems has a free 2D CAD they arre promoting. It reads my web address and trys to download in Japanese and then breaks, but I'll bet it is fine in America. The only concern is freeware is usually not ok for business use. There is other low priced CAD, though.
3D CAD is nice, but you would hate to add a $20,000 CAD license, I would think unless you are using it to drive a machine. Then, ProEngineer or other is said to be better.