Sounds like you've done your research. Yes on the M&P and yes on the Primary arms. I went a year getting to know my M&P OR before I started changing it. By then, I knew what kind of rail I wanted and what length. I also knew what stock I wanted and what set up I wanted to go with. I was very glad I waited and got to know my AR. I was surprised at how much I really liked the handgard it came with. You might be too. After a year, this is how it's set up now. I have around 2,500 rounds through it and haven't had a failure yet. They Sports have just as good of a reputation from everyone I know that owns one and many others.