Hi Benjamin-
Case building ain't easy until you do it full time, so be prepared for some frustration and re-work. Once you factor in what your time is worth you'd probably be better off buying (or consider it tuition at Casebuilders U), but frankly it's worth making a few to appreciate the craftsmanship of good case builders.
You might be able to purchase foam locally if you're willing to buy a lot. I think our local producer has a $250 minimum order now, but that re-foamed a pair of 52 input console cases. At any rate if you can justify the minimum it will be much cheaper than ordering from a case supplier. Ditto for the ABS, probably a big minimum order if you buy from a plastics wholesaler but cheaper per S.F.
Like Jim said, if you get serious about this, you'll be looking at auctions for CNC machines, air drive rivet tools, Cleko blind panel holders...
Making nice cases can be rewarding, but learn to make some cheap crap, too, so you'll have an income.
Have fun, good luck.
Tim Mc