Some questions to ask.
1) Are the parts Mil-Spec?
2) Are the barrels and chambers chrome lined?
3) Who makes the parts?
4) For flat-tops and railed fore-ends, are the rails Mil-Spec?
5) If a collapsible stock, who makes it? Is it aluminum? Is the buffer tube mil-spec? How many positions? Does it come with the buffer spring and buffer?
Avoid Number 1 Sales. (I think that's the correct name.) And be careful of the no name dealers sellin no name parts.
You can get kits pretty easily from Bushmaster, DPMS and other good companies. It may cost a little more, but you'll be happier in the long run.