I have been lurking around here for a while and have decided to build a rifle; actually 2. The wife said she wants one also. More on hers later.
I have decided on a 16 or 18 inch mid-length. Radical Firearms has their 16 in. SOCOM on sale this weekend, and AimSurplus is out of stock on their LOSOK 16 in.
Should I get the RF deal now or wait for the Aim one to get back in stock? It looks like RFs comes with a gas block and tube.
Which leads me to another question of what exactly do I need to get when i go to put the barrel on; nut, washers, etc.

What is your budget on these rifles and their intended use? Also, have you thought about the option of buying complete uppers? They usually come with warranties and always come with the peice of mind that they will run without a hitch, when gotten from a reputable source. When building mine, i went with a complete upper from BCM and assembled a stripped lower with hand-picked parts. Going that route allowed me to put more funds towards the rifle itself rather than investing in tools and fixtures. The lower only requires a couple basic tools to assemble.

Oh, and welcome to the community.

Right now I'd say this one is a budget build. I'm just getting my feet wet and learning as I go. I want to build it from scratch; not get an assembled upper, although it is tempting with some pricing I've seen.
The wifes  will probably be a heavy barrel bench rest type that will happen in the future.

The rifle builder that we have on this website is a great tool to use when looking at options. I have a Midlength that is on a 16 inch barrel because I can mount a bayonet to it.

http://www.brownells.com/.aspx/lid=11004/guntechdetail/How_to_Build_An_AR-15_Video

Additionally....

1) Use the approved moly grease on the barrel nut threads
2) Use barrel blocks when working on a muzzle device