I'd say a LPK and maybe a complete BCG. Only problem, they tend to morph into a new AR!
So why not cut to the chase and buy a spare BCM.
You know what they say about spares ... one is none and two is one.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek