Figured I'd build it, shoot it a little, and sell it at a profit -- and quickly realized that she wasn't going anywhere.
Loved it so much that I've also done an MR556 pistol build out of enthusiasm. I did end up selling that though, just to make room in the safe.
I used an ordinary BCM LPK on the MR556 build and had zero issues. Spikes "No-Logo" lowers remarked with HK416 markings. With an MR556 upper, your biggest hurdle will be to get it cut down properly. Best bet there is to find a used upper that has already been cut down by a reputable smith.
If you have the means -- the 416 is worth it. It will hold resale value much better, and even appreciate if you shop smart.
As for the binary trigger -- the Fostech Echo will not work. The Franklin trigger should be good to go. In either case, you'd be better served with a Geissele MR/416 trigger.
However, I will warn you off all of the above now. You have a P7M8. To pair it with a 416 as its 5.56 playmate just will not do. Since you hate money, uou need an HK93 instead. Then get a USP, or Mark 23 to pair with a 416.