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Solidworks is a parametric CAD program and there are a lot of other options. I would suggest starting with a free Onshape.com account. It's all online and they have a lot of really good training...

You can export a .stl file of machined stock and re-import it for use as a stock model. As usual with .stl files units are a pain to manage between export and import.

I find the in-process models...