There are plenty of free design programs.
I design in 2D using AutoCAD. Spent two years in school learning how to use it.
Blender 3D is a free program. Here on PM a fellow member did an ebook on using it to design models.
Here is a link:
New Free E-Book About Paper Model Design With Blender 3D
There are other freeware programs SketchUp is one. For editing there is GIMP and inkscape.
Hope this helps.
Mike