BUIS: Troy's are great even though I haven't used mine yet (have an ACOG)
OPTICS: Have the 512 on another gun and work well ~ not sure if the 522 is brand new or an old item ~ saw that it had an outer Reticle and was actually cheaper than the 512.
RAIL COVERS: I use both Magpul ladder and XT ~ the XT's fit very well.
VFG: Tango Down is as good as the rest. I have one but I put one from SOG Armory that has a rubber outer layer that is very comfortable so I haven't changed yet. I would put the TD on if I had a tac Light with a grip switch though.
PISTOL GRIP: Have the Hogue also have an unused Ergo ~ personally I like the feel and comfort of rubber over plastic.
MAGS: PMag all the way.
Your on the right track!