I have a very similar build to the OP, but with a Centurion 16" barrel.
https://lh6.googleusercontent.com/-s...o/IMG_0025.JPG
This has been my project gun over the years, originally conceived as a kind of "do all" carbine with a suite of optics in QD mounts. Very handy rifle, and I think you're on the right path. The Rainier EVO is a very nice handguard, but I am also very interested in the new BCM units. I've been mulling around a 20" government upper, but part of me just thinks the 18" is the way to go.