As far as which M14 to base it off of, I'd say "a cheap one". So maybe a second hand Cyma. There's an enormous amount of work going to be involved to achieve this though. Given that all you'll really have from the M14 is the gearbox, half the stock and maybe the feed/hop area, there's a whole lot of engineering coming your way!
Best of luck though, should be cool if you do get it done.