It's got to the point in the used market where you can buy either cheaper than build one. The major advantage to a build is you can make your choice of trigger. I built my wife a copy of the one she was issued in Germany in 1980 (Lance Missile crew in the Volga gap). The only 2 things that are different is it isn't select fire and has a Timmney drop in trigger. The drop ins are well worth the 200 bucks over the stock ones you get with the kit.
As for the stock, if you are use to Monte Carlo rifle stocks it is almost identical in function. Having the stock set lower it allows the scope to be mounted closer to the center bore of the barrel as any stock seton the recoil buffer..
You can pick up a DPMS AR10 right now for 750 a Armerlite is about 200 more but it is the company that invented them. Another thing about AR10 builds the parts aren't nearly as interchangeable as the AR 15 parts. There is a AR10 lower though that uses AR15 triggers. Bolt carrier groups for the 10s are 2xs as much as the 15s. With all the cartridge options available for the 15s that share the same bolt it's nuts. Just the ones that only require a upper are 204, 223, 6.8 SPC and 300 BO.
After shooting my sons I've been mulling around the thought of a 22-250 AR10. Since I reload I could use the same bullets I use for the 223 in the 22-250 but push them 1000 fps faster. Even that build it would be cheaper to buy a DPMS and just rebarrel it than build from scratch.