So with the great deal from Coleburn Armory on lower receivers, I have decided to build a Mk12/SPR style rifle (already have an AR for CQB, building this one for longer ranges and possibly competition).  Looking for some opinions on rail systems, upper receivers, and barrels: 

-Rail system: a true Mk12 clone would have the KAC RAS, which don't seem to be in stock anywhere, and are a little pricey.  I was leaning towards either the new KAC URX 3.2, the Apex handguards (which look sexy on all the Crusader weapons)

-Upper receiver: either a regular upper or the LAR OPS-4 with a left side charging handle (anybody have experience with the LAR?)

-Barrel: White Oak Armament stainless steel, 1:7 twist, seems to be the best price

Any other suggestions for a Mk12/SPR?  Anybody else built a similar rifle?