Hello Everyone - New guy here!

Been debating between buying S&W MP15 or build. Well, you all have inspired me to build my own. Researching parts and such, I'm planning 556 with 16" or 20" barrel (1/7), forged upper and lower. Haven't settled on specifics yet. My very basic question relates the small parts kit for the lower receiver. There are a bunch of different manufacturers, etc. What's the difference? Is any one higher quality than the rest? Looking for something that includes the trigger, but I'm sure I'll upgrade this later.

Thanks so much!

There are many different distributors (but very few manufacturers) of lower parts kits.  
Most online retailers buy from a manufacturer and then slap their name on them.  

For what its worth, I've had great luck with the Palmetto State LPK's.  Never had one that was out of spec.  
For a few dollars more, you can get their kit with the Enhanced Polished Trigger (EPT) and that will tide you over until you do a trigger upgrade.  

Here's one that also comes with a MagPul MOE grip and trigger guard for $64.99 - http://palmettostatearmory.com/psa-defender-moe-lower-parts-kit-no-ambi-safety.html

I had a PSA buffer retainer break, for some reason it looked different than others I have. It was their standard model.

I just replaced it, it wasn't worth dealing with CS for a part that cheap.

Complete PSA 16" Mid w/ MagPul FDE $450 shipped

or

Complete PSA lower w/ MagPul $150 shipped
+
Complete PSA 18" SS Free Float MLOK upper $350 shipped

cmmg builders kit + magpul k2 or bcm modwhatever if you want a more modern vertical grip + magpul trigger guard + alg act or qms

make sure you get a 7075 aluminum buffer tube, lots of 6061 garbage out there

Thank you for all the info! Been hearing a lot about PSA. Their parts kit seems reasonable. I like the slight upgrade. How are the assembled uppers? Are they accurate? I notice they have a flash hider. Anyone know if that is welded to the barrel. I live just outside the USA in NJ.

CMMG builders kit and ALG fcg are my go to.

PSA lpks are not what they used to be. I have heard/read several reports of breakage. Mag release and bolt  catch among the most.

Maybe look at the alg defense parts kit. I think they have a kit with their improved trigger. Geiselle/ALG build great parts so I'm sure their parts kit would be a good one.

Something like this. https://algdefense.com/complete-lower-parts-kit-with-act-trigger.html