FWIW, I just bought a 'kit' from Palmetto... everything except the lower itself... for $400 shipped. Spend another $50 or $60 or so on a stripped lower... and you can have another complete rifle. Having a complete rifle is a better option than swapping uppers, etc. I was surprised... the upper was completely assembled, I expected it to arrive as parts. A good cleaning, a once over on a few things... and off I went. 16" mid-length upper, MOE furniture.... nice rifle.
If you still want to just go the upper route, I would buy one with the complete BCG... don't swap bolts. I always recommend a mid-length gas system on 16" barrels... just my .02 worth, although I'll say most carbine length systems work well enough.
Having said that... here is a Daily Deal at Palmetto on a carbine-length... at $270 with BCG???
https://palmettostatearmory.com/ble...ato-1-8-phosphate-classic-upper-w-bcg-ch.html
Rifle on the left is the Palmetto kit...
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https://palmettostatearmory.com/psa...-1-7-nitride-moe-odg-rifle-kit-516444601.html