Well having a CNC is almost as good as a cheap 3d printer for what I was referring too, you'll just have to alter the designs a bit to be doable on the CNC.
When you say High Volume... do you mean Large Print Volume, or Fast Production of parts? You can make your 3d printer as big as you want, it just gets very complicated structurally. Speed wise, you can get fairly fast with an FDM printer, but I'd guess not as fast as you're thinking. I believe SLA is the way to go for speed, but I think you'll have issues building your own with that, and I don't think that's the type of 3d printing you want to get into, especially with your reservations on the cost of prototypes. Others would know more about SLA's.
I find it odd that you are finding HDPE and Aluminum to be cost prohibitive for prototyping. Ok, I understand having to pitch your 14th prototype becoming a PITA, but with modern CAD design, you should be able to most, if not all, the prototyping digitally. You should be able to get accurate measurements on the associated parts and design around that. It also depends on the size of the blank, so If it's a 20" cube of aluminum, yes, that'll be expensive. Have you considered other mediums for prototyping? Like layers of 3/4" ply laminated together to form the blank you need? It'll make a working prototype, barely, but will save you the cost of the final material. A cog or widget CNC'd out of wood would work well enough to prove proof of concept, may look like crap, but it should work.
But I do think building your own printer is a great way to go if you have the materials and means to do so. You will end up buying a lot more parts than you expect to, though. The rails, threaded rod, motors, controller board, belts, etc. There's a lot of stuff you won't have lying around your shop that can't be fabricated.
