Wi, that's a real good price OKdonk gave you for the Spike's stripped lower and Magpul STR/receiver extension and LPK from PSA. With the free shipping you get a great lower for $240 plus transfer fees.
For about $50 more you can get a blemished BCM lower but still have to buy a stock which can be another $50-100. I would get the lower first just in case since it could be the hardest part to buy if parts dry up around the election (but it's hard to say what will be out of stock months from now). The first lower option can give you a great lower for less, in fact it's the option I went with, then I put on a 16" LW BCM midlength upper and now have an excellent carbine.
Your plan of getting the upper minus BCG, etc. can break the price up for the most expensive part, maybe $400+ for the stripped URG then $250+ for the rest if you go Magpul furniture/BUIS. Then maybe a sling, magazines and ammo.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek