Hi guys, not trying to be contrary, but you actually CAN complete one of these with a drill press and a dremel as long as you have a good jig. It is not, however easy contrary to what youtube may make it seem like and it takes a while and can be extremely frustrating or even dangerous, but then again maybe I'm just not that talented
There are several sites online that have jigs. A mill makes things infinitely easier and the finished product looks a lot better, but it's not 100% necessary, there's actually a guy online that did one with a hand drill... google mujahadeen ar15 build I think. Anyhow, it's not for the faint of heart, and the total cost with jigs and bits and all will be alot more expensive than buying a complete lower unless you build a ton but all that being said building it that way made me feel pretty good about myself and was fun for the most part. One thing to be aware of is depending on where you get your 80% receiver you may have to tweak it to get the mil spec uppers and buffers tubes etc. to fit.
As far as the serialization goes, you are not required to put any markings at all, however it would probably be a good idea to at least put manufacture place and date and any number on there just in case to prevent questions at the range or en route. (there's a nice tutorial on self anodizing and etching by a guy named VADER SPADER on another forum. I really want to try this once I have time as mine is still blank aluminum.)
These are completely legal according to BATF as long as you abide by state and local laws, and don't intend to sell it. That being said
I'm not a lawyer so check for yourself. Happy New Year by the way!