The comment about the plastic delta ring is an inconsequential matter. It just retains the hand guards. It's not under any significant stress.
I built a Del-Ton 20" frankenrifle from their kit on a Spikes stripped lower. Basically basic parts all. The rifle has performed flawlessly and totally satisfies my accuracy requirements. I built an -A4gery, flat top receiver, with a removable A2 carry handle / rear sight. Iron sights and practice and I'm hitting a 14" steel target at 400yds with regularity, using XM855 and my own 62gr FMJ reloads, using the fundamentals of rifle marksmanship taught to me in the Marines ('87-'93).
It really takes a combination of out-of-spec parts to make an AR-pattern rifle misbehave. Beyond that it's more about the shooter than anything else. So find a good value on components, look out for bad reviews on technical glitches, and use the money you save to shoot shoot shoot and hone your marksmanship.
Palmetto State Armory has good deals on basic 16" and 20" rifle kits and often has blowout deals on 'cosmetically blemished' stripped lowers. Together you can build a rifle from them for about $500 right now. and pretty much ~$600 anywhere else. Gun sales (as figured by FBI NICS monthly data) are still high as ever, and it seems the ar-15 market is full of enough vendors that availability and price have both greatly improved the last couple years. No is the best time to buy in the last five years.
Lastly there's only about 5 actual manufacturers of lower receivers, with most of the 'brands' putting their labels on the same-sourced part. So don't pay much heed to the brand wars.
ar15.com's armory and technical forums have ALL the info you could desire on both the hardware and assembly. It can be done in an hour with a few basic hand tools. Avoid their General Discussion forum if you are seeking technical advice. It is a silly place.
