I've built 4 systems, including a recent 300bo pistol from PSA parts and they've been GTG with one minor exception. And that's with that particular configuration. The sling plate was too tall to allow the charging handled to ride over. I had to grind about a few thou off the top to flatten it out a bit.
FWIW their so called blemished uppers and lowers? I bought a blem upper for this build and all it meant was the anodizing was more a very dark grey than a black. The funny thing was, a quick swipe with 2 cents worth of frog lube and it matched my lower exactly saving me $20 on this build.
Note there's no crown nut so replacing the sling plate might possibly be a little, not a lot but a little, difficult because the thickness of the plate and the threading on the pistol tube is what stops it at the right position it to hold down the buffer retaining pin. The tube is also what holds the plate against the lower, there is no crown nut to adjust the tube in and out or hold the sling plate in position.
And with no crown nut you may want to use some blue loctite on the tube threads or a strap wrench to torque it down a little more than hand tight.
The sig brace fits. It is very tight though which isn't a bad thing. As in tight enough it's put the muzzle on the floor and really lean on the brace to with a lot of weight to get it moving after the first 3 inches. Tight enough you shouldn't have to use an spacer if you don't want the arm brace all the way on. There's very little chance it's gong to move on you, at least at first, the rubber might get warm during summers and soften up.