[Metternich]

Legalities aside (something you should seriously consider as it would be silly going to jail for selling and making toy guns)
I'd be interested if the price was below 600, Ideally in the under 400 range. I've contemplated the feasibility similar projects. Typically submachineguns from the mid-late 40's to the 50's use some pretty simple manufacturing techniques to make their bodies, and potentially could be easy to replicate if given the right tools material and information. Other guns that I've thought about were the Swedish Model K, Beretta M12 and the Sterling. Not to discourage you or anything, but the Sten is a little tight for space, are you sure you'll be able to fit a mechbox in that thing? What about a battery?