M4 or MP5 is really just a matter of preference, the difference in barrel length is small, and parts and upgrades are easy to get for both, slightly easier for the M4 but only slightly.
I definitely agree with several of the above, full stock all the way. Large battery ftw. That said, if you're a small guy a full stock might not work for you.
As to mounting surfaces, you're probably going to want to pimp out your MP5 so yeah, a SD is probably not what you're after. As the quote goes: "If there's a tactical need for it, the mp5 has an accessory to do it, and the more you put on it, the better it looks."
Metal bodies can be acquired fairly easily, there's at least two in the classifieds right now. But they're not cheap (think at least $150), so if you're going to want a metal body then you might as well just get one of the brands that come with a metal body. I'm an M4 man myself so I can't really tell you which mp5 manufacturers make ones with metal bodies, though since I'm also a big CA fan I can tell you they do. G&P probably does as well and possibly ICS. If all you want to do is take care of the clear though, a can of flat black Krylon spray paint works just as well.
Of course acquiring either a metal body or any guns by the manufacturers I mentioned above would require you to get age verified.