I started building my first ar lower like 10 yrs ago. Purchased the upper complete. Was pretty easy but made lots of mistakes. The mistakes were in my opinion part of the learning process. After lots of trial and error here is what I learned...
Get the right tools for the job
Do your research
Purchase quality parts
Research pros cons of major parts
Do listen to specs like torque and Molly grease
Have fun and don’t get in a rush
It’s a challenge but if you like building things for fun it’s really rewarding and helps you understand how the gun works etc.
I am not a fan of low quality firearms. I'd rather have 1 quality gun that 2-3 crappy guns, even if I just intended to punch paper. That line of thinking gets me in trouble, I end up selling a bunch of quality semi auto guns and consolidate them into another machine gun... the new machine gun itch is getting strong right now.
Not exactly sure what kind of kits you are asking about but here it goes.......
For complete uppers I have bought from Palmetto State armory before and really don't have any issues with their complete uppers they all seem to work fine. I have a 10.5" 5.56 upper from them and a 7" AR9 upper and have no issues with them.
For lowers I put my own together with LPKs and and stripped lowers. For the lower part kit I have had no issues with the cheap Anderson arms kits and mil-spec stock and buffer tubes. I usually upgrade the trigger though to something a little nicer I have a RRA NM trigger and a LaRue MBT.
So far all my uppers and lowers work well and go bang when I pull the trigger. They may not be tier 1 operator equipment that requires a manly beard and shemagh to operate but they work for what I want to do with them.
if you were to build a varmint rifle, i would suggest the giselle trigger with a 1/7 or 1/8 twist barrel to stabilize heavier bullets of 20" or more. a rail system or adapter for a bipod is all but a necessity, unless you like to lug around sand bags or other forms of rests. i prefer to build gas piston ARs over gas impingment, but that is added expense. to give a specific recipe of how to build the perfect rifle is going to be all but impossible. because what is perfect for you may not be perfect for me.