Nice build! Spikes is the only way I go. What made you go with the complete upper? Building the upper is where the fun is at
The Nikon is pretty nice depending on what you are planning on doing with your rifle...A good red dot in the P223 price range is the Vortex Strikefire...it comes with a 3X screw on magnifier and I have found it to be really versatile out to 300 yds... Cheers!