You don't usually need an H buffer in the 11.5" gun unless the port is really large. My first 2 AR15's purchased after learning on a Colt Sporter II were both Bushmaster 11.5" with the same heavy barrel/5.5" flash hider you have, and they ran like champs in high volume.
You're basically working with a 1980's to early 1990's configuration, so I would consider one of the ARMS mounts for the carry handle, and then place the RDS on that, and get a Delta cheek piece with a standard CAR Fiberlite stock. Since you already have it Magpul'd out, consider just using a fiber optic front sight post, as I have done on my 727.
+1 for the Right Hand sling attachment on the stock. If you are RH, place the attachment point on the right side. Your ACS should have come with a QD socket that is probably on the left right now. You are pretty limited in front sling attachment points with the carbine handguards.
+1 on taking plenty of mags so that you can focus on shooting and learning, not loading and getting distracted from the course. Take a good dump bag as well, and a Camel Bak. Start hydrating now, and have plenty of snacks after washing your hands from lead residue. I highly recommend Probar meal replacements.
Have good ear pro and protection from the elements. Kneepads are usually a good idea. Work out your sling issues before you go to the course. Study Green Eyes and Black Rifles from Viking Tactics, and learn how to set your sling up correctly for your height/chest build.
Green Eyes & Black Rifles - Autographed By Kyle Lamb | 5.11 Tactical