I think Pyree is right on the cases, they would be pretty straight forward and have a much better profit margin than most other parts. You would need access to a brake for metal bending, hydraulic shears to get large smooth cuts, and a way to cut out holes in the case, a milling machine would be preferable but there are other options.
For not involving incredibly elaborate technology, i think it is rather silly that we have yet to come out with great cases at reasonable prices and for custom cases you could probably make a killing if they were well designed.