The D-ring on a brand new rifle is a bitch to get down far enough to unseat the original guards. It's really a two person job, but I did it by using both hands (STS) to push down the guard and a flathead screwdriver in my mouth to push one guard off. I guess it you have gorilla strength, that's NA. I don't.
Getting the MOE hand guards on is challenging as well for the same reason, but there's also some tips on the internet about it. You basically need a rubber hammer to tap them into place.
WRT scopes - I have a Trijicon 4 x 32 on my Bushmaster XM-15 and a UTG 30MM 3-12x 44 Compact IE Scope w/Glass Reticle on my M&P-10. I paid $150, but apparently Amazon has them for $100. Seems to be a badass scope, and it is listed by S&W as "recommended accessories" for the M&P-10.