How accurately do you plan on replicating your rifle? The Marines' SAM-R and Army SDM-R have some notable differences if you plan on doing something more authentic than "in that style"?
That would be the first thing to decide.
Unlike the Marines' M16A4, the SAM-R uses the KAC free float RAS; the SDM-R uses a Daniel Defense M4 12.0 free floating rail. While not majorly different at first glance, there are some noticeable differences (hence, how much of a stickler for accuracy are you?)
Also if you're starting with the SPR (which uses the KAC rail), you'll also need a new outer barrel (20").
There's a bunch of other details you may or may not wish to pay attention to.