I'd advise against putting a RIS/RAS on it unless you plan to actually stick stuff on it. There are barrel mounts (clamps to barrel) that allows you to mount a bi-pod. Sniper rifles end up very front heavy when you start putting a lot of crap on it and it will feel a lot heavier than it actually is.
Folding sights aren't really required -- if you use raised scope rings, you'll never see your front sight. You also might want to replace the pistol grip for comfort, since as I said -- these things tend to get front heavy and I find the stock M4/M16 grip gives me blisters after a while.
The PRS looks a bit out of place because it's almost as long as the entire front end. I actually had a similar idea (mod M4 into a sniper rifle), though I eventually decided to just buy an M16 to do it (otherwise you will end up replacing the entire front end just for length).
(Years ago slapping a scope+bi-pod onto my M4... unsatisfactory)
Below: Current M4 and M16.