http://www.heckler-koch.de/HKWebText.../1928/337/4/19
Yes H&K does sell groups that will retro onto a lower receiver. Follow the link and check the pictures
I'm pretty sure in this world we're visiting now some companies make a dress up kit to convert a regular M4. You wouldn't have their grip, stock, and trades on your lower and that really seems to bug some people if they're not 100%.
From further digging there are some on the horizon but they're newer so not many have entered airsoft service. The VFC 416 which is nice but expensive and harder to find now and the JG/Echo which is also new and by reports surprisingly nice and being snapped up by anyone who can sniff one out.
I'm waiting on a JG myself. I have the VFC SCAR so I didn't want to dump another wad on a "backup" at this point. If I really like the JG then I'd get a VFC but I suspect I'd ditch the PEQ battery box.