I built a AR15-A2 from a Model1 parts set in 1989. Back then they got their parts from US military contractor overruns. The only change I did was to add a Wilson match barrel and a set of National Match sights. I still have that rifle and it is as reliable as any AR type rifle. It still shoots sub MOA, so I would say it has been a pretty good rifle.
I built another back in 2010 using one of their 5.56 upper assemblies and lower parts kits. It is accurate and reliable. I have no idea where they get their parts from, but they seem to be of good quality.
My personal opinion is that if you want a 7.62X39 rifle in order to shoot cheap ammo, get an AK or SKS. They are designed to shoot steel cased ammo and generally seem to cost as much as an AR upper. I have three Russian SKS rifles and the most I paid was $480 for a pristine unfired Tula. I bought another for less than $400, there are many others out there that are less than that if you look around. The last AK I bought (two months ago) was just under six bills and has the advantage of there being cheap military surplus magazines available. While they are not tack drivers, they are a hoot to plink with and I have found that either a AK or an SKS goes to the range with me more than any other gun I own.