I'm planning to go about the same direction as you. Have 1 gun to use as AR and DM. From what i've seen i need a few things:
Another lower receiver, doenst need to be from SCAR-H, another just like the one i have will do it.
Another gearbox modified internally to be as sillent as possible (need some ideas on that part)
Another hop-up unit (need to know if i can fit any but the original from the gun) for the long barrel, an if possible another one for the CQC barrel.
An SV outer barrel and the matching inner barrel (not sure of the lenght here)
So far the only thing i've seen