The program I believe fits your requirements best is simply called FreeCAD >
This has been around about ten years and can draft in 2D and model in 3D. It is open source, and can produce STEP and STL files which instruct CNC routers, laser cutting and so on. If you have the tools, you could produce your sheet metal patterns and cut the parts automatically from your drawings.
I haven't used FreeCAD, but it seems to be industrial design oriented (it appears to tracks object history a bit like Solidworks) than Sketchup and has some solid modeling and graphic ability. The interface looks to be fairly intuitive- as is Sketchup and the system requirements are modest. As I say, I haven't tried it and don't know anyone using it, but the results look promising and at that price, it's worth having a look/ giving it a try.
Other possibilities > Sketchup free version, or another tactic, if you knew how to use them and the timing is sufficient, would be have a 30-day trial of many great programs- AutoCad, Autodesk Inventor, and I think Rhinoceros. One of the best programs for sheet metal is Solidworks which can make cutting patterns from 3D objects and even place multiple parts for the most efficient use of a sheet, but very expensive and complex to use fully. I think my next industrial design program may be Rhinoceros- very capable, but not overly complex to use.
Good luck with your project.
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