Yeah the real reason you never get guides to this stuff is that when it comes down to making a successful prototype, what makes it good vs. just barely work or not work at all is somewhat of a black art. There is so much intuition and experience that goes behind decisions like what material to use, what kind of fastening, how to seal it, finish, how/where to route cables, what kind of cables, connectors, will-it-pass regulation XYZ, is it safe, etc. It's nigh impossible to really get all that into a guide, you really just have to go at it and learn (which includes reading what guides there are ;-).

Anyhow here are a few points that might be relevant to you:

- As someone else mentioned, unless you're in a money-is-no-object sort of group, usually at a large corporation like google, then you'll almost always start with some stock enclosure for testing purposes. There are tons of choices, from extruded aluminum to cast steel, bent sheet metal, ABS weather proof housings. The list goes on. No, it's not a custom box designed by Dang from Silicon Valley, but trust me there's crap you didn't think of yet for the stuff inside the box. One step at a time, and remember, Compromise is the Hypotenuse of the Conjoined Triangles of success.

- Next stop: Proto Case[0]. They will use your CAD, or you can use their custom 'box cad' package. They will do sheet metal, solid machined metal, powder coating, fasteners, everything. Not ultra cheap or anything but still a good deal if you value your time. I can't tell you how many hours I've spent fixing/remaking sheet metal enclosures when I accidentally bent a flange 2mm too long, or screwed up dimension scaling while waterjetting it. It takes time to become proficient, even if you have the tools.

- For 3D printing and such, there are tons of 'Maker' shops that have popped up over the years. I've used Ponoko[1] before, and while slightly slow and again not ultra cheap, the parts are nice. They do 3D printing of many sorts (ABS, PLA, sintered metal, powder ceramic, UV resin), as well as laser cutting. Note: Laser cutters are REALLY awesome for prototyping things. I use a laser cutter constantly when developing prototypes.

- For machining, check out the First Cut[2] service from protolabs. You send them a CAD model, they analyze it and then you use a flash plugin to select materials, tools, threading, etc. and click go. Last time I used them they did single approach 3-axis parts in Stainless, aluminum, mild steel, various plastics, and brass. They also do CNC turned parts (on a lathe). The parts are very high quality for the price. Beware: The tolerances are not super tight, and they won't make any guarantees about mating parts! They made parts that mate for me and it worked, but it's a bit of a gamble.

- Protolabs also does 3D printing, as well as small run injection molding[3]. I haven't used this service so I can't say much about it, other than that their advertised prices are significantly cheaper than getting tooling made and certified anywhere else.

- Locally vs. China? Well if you mean getting a production run done in Shenzen, then if you have to as the answer is probably 'local'.

Anyhow, this is just based on my experience. I'm by no means a manufacturing or design expert. But I've made and worked on countless prototypes for all sorts of applications (nuclear medicine, MRI imaging, thoracic surgery, industrial microscopy, POC blood testing) and what I've said has held true in my arena. One final hint: If you're thinking of taking some hardware device 'to market', stop and think HARD about who your market is, how big it is, and how much it will cost to get there. It's quite possible that a traditionally 'production run' isn't necessary to get started. Lots of hardware projects started small. Also, in my experience investors aren't too keen to accept risk of the 'will the prototype work', or 'do you have the expertise to scale the hardware' variety. So bear that in mind.

[0] http://www.protocase.com/products/electronic-enclosures/

[1] https://www.ponoko.com/

[2] https://www.protolabs.com/cnc-machining/

[3] https://www.protolabs.com/injection-molding/