-PSA upper
-BCM BCG/charging handle
-A reliable stripped lower receiver, pick from hundreds and I am a supporter of polymers
-Daniel Defense lower parts kit, if you can find it...or Spike's Tactical, Stag Arms, PSA...
- Magpul hardware, stock, grips, folding rear site (MBUS) if that's your cup of tea
- Magpul pmag magazines
- Vortex Strikefire red dot optics
You can upgrade to a Rainier Arms lower receiver if you have time (extremely slow and not great customer service) but excellent lower. Or your optics to Eotech. Or your trigger to a Geissele SSA trigger kit.