We had a big discussion about this on CO4x4.org just yesterday...
Building your own is the way to go.
I built a Del-Ton 2 years ago and love it. Only took me about two hours and I am NOT the most mechanically inclined person in the world!
Here she is:
I went with a mid-length upper (Middie) for two reasons: 1. Longer sight radius (I use iron sights) and 2. longer gas tube = less violent cycling.
As far as components, I haven't checked but I'd be willing to bet that most of the "manufacturers" out there probably only actually build uppers and lower receivers - my guess is that everything else (springs, bolt/carrier, sights, furniture, even barrels) are probably sourced from the open market. Meaning that if you pay top $$ for a "premium" gun you're paying a lot for a name. And as far as the quality of lowers and uppers, I think that as long as they're in spec, they're probably all more or less functionally equal, at least to the extent that any civilian shooter would need.
I got my Del-Ton lower from JSE Surplus for about $90 and the rest of the kid I ordered directly from Del-Ton. I know Del-Ton has a reputation for being a "budget" gun but for my needs it's perfectly adequate. YMMV.
EDITED TO ADD: I should point out that my "kit" included a fully assembled upper, so all I had to "assemble" was the lower receiver and stock.