I'm a huge open-source guy(my Peakd is mostly about that), and my ex-GF was Revit/AutoCAD designer. Unfortunately, CAD is an area where open-source is pretty lacking which I found out when we were looking at something she could use that was open-source.

Blender is a top-notch opensource project, and it can do many things. Some sadistic people even use it for a video editor, but in my experience the learning-curve for Blender is pretty high.

You mentioned you can mill. Do you have a CAD/CNC Mill? My father's a welder/fabricator originally by trade. He has mills, lathes, ironworker machines but they're all ancient and pre-CNC/CAD. His Bridgeport mill is from the 60's lol.

EDIT: I forgot to mention, Inkscape. It's no good for 3D stuff, but lots of people use it for 2D cutting CNC work. As a visual designer, it's awesome free software that's on par with Adobe's Illustrator.