For anyone contemplating a build vs buy I'll give you the rundown on a rig I built for a guy I work with.

Midlength Cav Arms 16" upper:  In case you didnt know the barrel is by Sabre defense and chrome lined 1/9.   The steel is the same used for 50M2 barrels, very tough stuff.  IMHO a step above bushmaster and possibly colts.

Stag lower,  he overpaid for it about $30ish.  I could have found him a better price.

DPMS LPK from brownells.

6 Position Cav Arms stock
ARMS #40A2 buis  ( damned nice sight btw)
single point sling adapter
single point sling
Right at around $800 total.    

Not considerably cheaper than you can buy a factory carbine for but in a configuration you cannot get.   Midlength flattop with a very nice flip up buis and ready for optics.  

I highly reccomend a build over buying factory.   He could have saved a few bucks here and there but it's still not bad for a 100% new rifle.