Quoted:
Ok so I am on my first build and relatively new to the AR-15. I figure no better way to really learn a gun than to build it. Picked up a CMMG stripped lower at my local shop for $150 this week. I have been reading on this forum for quite a while but I have not quite rapped my head around the best order to purchase the upper in.

1.  As far as I can tell the upper receiver assembly is pretty much your A1-A4 selection. Also read not much difference between the different brands of lowers. That it is more of a visual preference. Is this true of upper receivers also? No, the Lower Receiver = Low Stress.  Upper Receiver = High Stress (i.e. Lots of moving parts). That being said, it is important but not always necessary to buy a quality upper receiver. Depends on what the purpose of the rifle is (Hunting, Target, SHTF, CC, etc..)(i.e. Essential Arms << LaRue)I would rather spend more $ on the barrel for accuracy if the assembly doesn't make much difference performance-wise.
2.  I am not quite sure I grasp M4 cut. You can use a M4 barrel with a Regular receiver, but not vice versa? If that's true what factors would warrant M4 cuts? And as I am new to the AR-15 what are most barrels today? This post will explain in better details of M4 feed ramps.

I suppose a matched barrel assembly would be the answer but I prefer to buy separate for value/defray cost and to customize as I want it. Thanks so much for any advice!