I recently purchased an M&P15 Sport, and wanted to install a picatinny forend rail to go on the stock forend. I purchased the carbine-length ProMag Aluminum Rail (PM003D). The rail has a length of 4.625" with mounting points on 3.786" centers. Unfortunately, that doesn't space out well on my...
The Primary Arms optics are good. I'd also recommend that you look into the Vortex Optics. I am very happy with my Strikefire, and see that they have just come out with the Strikefire II:
Vortex Optics - Red Dots
You really can't go wrong with either PA or Vortex.