Comment by deanfranks

Take a look at Autodesk Fusion 360. It is free to makers / small startups. I use it extensively for product prototype development including for 3d printing and subtractive machining. Another good resource is McMaster Carr, they have downloadable 3d models for most of what they sell. Having accurate 3d models for all the fasteners and other parts makes a big difference in quickly building a 3d model as well as the quality of renderings done in the cad system.