Update.  I'm doing quite a bit of research on different brands of parts, based on some of the adivice I'm getting here, as well as some Youtube videos. I know... I should treat those with a grain of salt, but some are pretty good.
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					Originally Posted by  Grizzman
					 
				 
				If I were to start a fantastic build now, I'd try really hard to snag a Geissele Super Duty Receiver Set tomorrow for $157. 
 
Yep, that's more than an Aero or Anderson, but you're worth it.     
			
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I agree.  And I'm focused on Geissele for the trigger group as well, per AFJuvat.  
What are y'alls thoughts on "Sharps Rifle Co." for BCGs?    I'm seeing a lot of good videos on their BCGs (eg XPB series, or the "Extreme BCG).  Here are a couple of videos I've seen...
     
      
     
      
Now, a question or two.   I'm seeing a lot of videos on "Buffers" and "Buffer weights..."   For example:
     
      
and
     
      
I gather that they help balance against the gas system pressure for the right extraction'/ejection force, but I'm not sure how it works. Do they work with the BCG, or with the gas system in the upper?  Or something else?
Question 2:  Headspace.... How important is to get headspace right? I gather pretty important in avoiding a catastrophic failure...
     
      
Anyway, I should be ready to start buying parts (at least the upper/lowers) by mid January.We'll work on barrels and gas systems, next.
Thanks all!