If costs are a concern, Bravo Company has forged M4 milspec demo blemished uppers for $70 with EPC and FA. AIM Surplus has Surplus Arms and Ammo stripped lowers for $60, also forged and milspec.
LPKs can be had for $40-$50 from many ARFCOM industry partners.
Great barrels can be had relatively cheaply from AR Performance, and Bravo Company as well as Daniel Defense and YHM.
Bolts and Carriers from Bravo Company and Spikes Tactical (who also sells a very nice stripped lower of which I have many) as well as buffers and gas blocks and tubes can be had at very decent prices.
Stocks from Mag-Pul and VLTOR as well as standard A2 are available from Midway or vendors on ARFCOM.
Forend, again Midway, Brownells, BCM... many ARFCOM vendors.
Charging handles are from $20 to $90. BCM (Bravo Company) has a nice Gunfighter model which is getting rave reviews, but I swear by the PRI Gasbuster, but they are darned expensive at $82 dealer.
Seriously, go over to ARFCOM (ar15.com) and look around. Also check out their Equipment Exchange. You can get many used items and save a ton as well as see what many of the industry partners have for sale on the boards.
In short, if you cannot get it through the sellers on ARFCOM, you don't need it.
And you won't need headspace gauges. These are not bolt actions. The headspace is set on the barrel when the barrel extension is put on. Like I have said earlier, I have build two dozen ARs and LRs and I have bought all the gauges. I have yet to find any that had excessive headspace and only one that would not close on the GO gauge. And while theoretically I might someday find ammo on the large side of SAMMI (or NATO) spec and it may not chamber, that day has not come. It is a rare bird that will not headspace correctly, especially in 5.56.
And don't be in a rush to buy a barrel nut wrench. Many forend makers have proprietary barrel nuts and you will need their wrench to put them on, so hold off until you know what you need.
You will need an upper receiver block. I like the two-pin type by DPMS or Frankfort Arsenal (Midway sells both) and they will hold ANY upper. The clamshell type will not hold many uppers such as the VLTOR or DPMS LO-Pro or other odd shaped uppers. The two-pin will hold them all. Roll pin holders and punches are a great help, also available from Midway and Brownells. You really do not need a lower receiver block. I bought a Flipper block when I first started and have yet to use it.
