You are right about those DOE kits being valuable, even the clones are stupidly priced. The friend of mine that I bought my M16A2 from told me not to put the 9 mm uppers on my registered receiver. He said the recoil impulse will egg out the pin holes. Not sure on you DOE kit but this 635 9 mm kit came with a stainless steel hammer & pins set. My buddy said the regular M16 hammer will break as would regular pins. The stainless steel pins are harder & are the cause of the egging on the holes.
He also recommended using a Quarter Circle 10 lower to build a SBR on. Not sure if any other PCC lowers were available at the time he built his. He said the feed ramp on QC10 works with the Colt magazines. He also had a war with Allied Armament over their drum that never worked correctly. Colt marked 9mm magazines that are LEO marked are something else that is stupidly over priced. I went with Metalform magazines as that's what QC10 sells & my friend say work reliably.
Blue Mountain Supply has a really great price on their Colt LE6945 10.3" CQB Monolithic Upper.
I bought one new for my M16 & it was like $1,200.00 from Specialized Armament years ago. I really love that upper, but it's heavy.