That's a terrible price for a DPMS kit. And they may call it milspec but DPMS usually sells a commercial kit, which is what appears to be pictured there. If you can find anyone who has stock, try to get a milspec kit. PSA and BCM used to have much better kits for less money. PSA even had a kit with a MOE or CTR stock for not much more than that thing at Brownells. I know it's a bad time to be looking but don't settle on high priced junk.
As Sputnik said, a standard stock wrench will work on rifle and carbine receiver extensions.
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