Manufacturing a device from off-the-shelf components
are there any ready-made device bodies that can be purchased?


Yes.  The search phrase would be "electronic enclosures", or "enclosures" if you're already within an electronics supply house's site, like digikey.  This should tell you who the big players are, and then you can search the manufacturers site for semi-custom options if your quantities are big enough to make that path worth it.


  is it possible to avoid any custom-designed fabs and just assemble such a device from off-the-shelf components?


Again, yes.  Anything is possible.  There are many forms of single-board computers that you can center the prototype around, with all sorts of form factors, that will let you do this.  The question, of course, is "is this the right way to go?"  If you're shooting for a proof-of-concept prototype with a "does this work" type of answer, it makes little difference.  If you're prototyping to see if you can make it fit into a certain required form factor or size, the boards you select will have a huge impact.  If you're really prototyping so as to scale up to production volumes, now you're entering a "design for manufacturability" stage.  

Of course, there's the matter of what your sale price needs to be, and can your manufacturing costs enable that price. Find your boards, and find out what sort of licensing and OEM agreements you can reach with the manufacturer.  You want to consider what will happen if your supplier goes out of business -- can you make your product live on?  There are obvious tradeoffs to having other manufacturers of value-added products beyond simple components in your supply chain.  You can get off the ground faster and less expensively, with less design effort, but cost may be higher (or lower) and you can introduce instabilities.

I recommend you take a gander through Ulrich and Eppinger, Product Design and Development, for a quick intro to some of these topics.  If this is a business venture of some importance, you might want to scrape up budget for a consultant who can help your product meet your needs.