I think that I'm going to jump straight in and go for an M16 or something similar to that.
There's nothing against changing out the stock/upper/grip etc on an M16 once you purchase it is there?
Generally speaking ( and other SME's please chime in)....The nice thing about a machine gun, it that it trumps all.. So you can put a shorty upper on it, in a variety of calibers, change stocks, grips, whatever you want. The only odd ball was that BRP made an MG-42 upper that was determined to be a firearm by the ATF (dont ask me how), so you could not slap it on a Full-Auto lower. I have several uppers including a belt-fed upper for my M-16 lower. Plan on about 18-25K depending on conversion/model/factory variables. I have an older Colt SP1 conversion and had a small pin conversion done to the forward pin (to remove the goofy old SP1 large Pin) by M60 Joe. It allows the use on Mil spec uppers on an SP1 lower without a special adapter.