I've built a few. I just built a Grendel the night before last. To do it with bare parts, you'll need quite a few tools, such as a special wrench and special vice block, and the roll pin tools are nice to have. If you plan on pinning the front sight/gas block, you'll need a drill press with a vice or a mill. The worst thing you can do is not have the minimum needed tools for the job. The Colt armorer manual is helpful.
As far as parts, I can say that as long as you get mil spec stuff you are okay. DPMS has good parts. Their trigger group is machined so you can polish it up real nice and get a crisp 4lb. trigger. The A2 buttstock has a metal trapdoor (the one I just got did). For receivers, I get those locally. They make a lot of them around here. Aero Precision makes an okay one for cheap, I fitted mine myself at the store and there is NO play, but at the same time, you don't need a hammer to pop it open. Just right.
Barrels? Wow. When it comes to barrels, I like to get a fine one. The barrel is what really makes or breaks an AR. I just got a Satern. If I were to go with a shooter, just a range toy or even a fighting weapon that doesn't need crucial accuracy, I'd get a chrome lined barrel. They just last longer. But my HD weapon, an M4 I've had since '02, is built on mostly Olympic Arms parts (save the trigger --their triggers are AWFUL). I built using the cheapest parts I could find at the time. The barrel is blackened stainless. Oly, they don't have the best reputation, and I would never use their parts exclusively, but this thing runs like a champ. Their receivers are actually not that bad, the bolt components work fine but are poorly machined.
My advice to you is unless you plan on building a few special weapons, is to get your upper assembled by the factory. The tools add up and collect dust. I have 'em, but then again, I build stuff I can't buy that way. If you have a buddy that has those tools, well, go that route...
For most of my builds, I like to use the KAC FF tube, custom barrels, JP Ent. tactical bolt carrier (by FAR the finest bolt carrier made) and the rest mil spec. I might use a two stage trigger, but if I use a single stage, I modify a DPMS one to my liking.
There are places online, I haven't dealt with them, that are supposed to have all mil spec kits that contain parts from military contractors. The parts can be Colt/FN/Somebody else, or all from one place. It wouldn't matter to me really provided it is all mil spec, that is what you want anyway. Top that off with a good barrel, and you have a weapon every bit as good as the high price brand names.
Sometimes it seems cheaper though to just buy a used Colt and then get whatever upper you want later and then have the two. You'll get a good lower this way, and then you can get that Noveske upper later (if that is what you want). Colt makes a good weapon, I used one for years and could hit point targets to 600m. That was with a chrome lined barrel, 14.5, narrow profile, the older cheaper collapsing stock, and non FF rail. And factory iron sights.
