I'd get a BCM Midlength upper from Bravo Company (G&R and Denny's carry them also). I don't know if they'll be back in stock soon but they're worth the wait. Get an M16 carrier with a proper MPI bolt. Make sure it's gas carrier key is staked right.
For a complete lower, I would get one of the fully assembled LMT or Stag lowers. The Stags have the quality CMT M4 stock w/ in-spec sized buffer tube, I believe, and the LMTs can be had with an upgraded grip and SOPMOD/Crane type stock if you want.
That's how I'd go about getting an all-new midlength. You could always add the BCM upper to a proven lower you already own and trust, too.
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