I don't buy SBRs. I build them by buying one part at a time as I can afford them. It allows me to build quality guns without hurting my pocketbook.
You should take my approach to building an SBR or any other AR rifle; You first buy all the parts to build it yourself, but you just can't resist getting an el-cheapo hand guard, a so-so trigger, and mediocre glass for the optic. Or, you paint a really nice hand guard with Brownells Alumahyde, but it goes on too thick and looks like total sh*t and the color is butt ugly.
And just to add insult to injury, the castle nut in that quality chinesium buffer tube kit you got on a Black Friday deal does not have any staking notches.
Then, you drop more money to replace all the parts with what you should have bought in the first place.
Anyways, that's how I do it.