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I'm not sure what almostfree means by a lightweight barrel

A lightweight profile barrel is the same as a "pencil" barrel or 0.625" in diameter, which is the same diameter that was used on the M16A1 / XM177 series of rifles. Unless you plan on doing a lot of sustained rapid fire with the rifle, I don't think you will see much in the way of practical difference between a lightweight profile and an M4 profile on a general purpose semi-automatic rifle. The pencil profile heats up more quickly, but also cools more quickly. The heavier profile barrels heat up less quickly but also cool less quickly. It is my opinion that the pencil profile barrel handles a little bit better (and I have no need for the M203 grenade launcher mounting ring on the M4 profile), though the difference is much more noticeable on a 20" rifle than it is on a 16" carbine.

The topic of barrel profiles gets debated quite frequently on AR15.com. I'd say read a lot about it and make your decision based on what you want out of the rifle. I'm obviously biased towards the pencil profile and my favorite AR-15 is one I built from a surplus M16A1 upper.

http://www.ar15.com/archive/topic.html? ... 8&t=313054" onclick="window.open(this.href);return false;
http://www.ar15.com/forums/t_3_118/398076_.html" onclick="window.open(this.href);return false;
http://beta.ar15.com/archive/topic.html ... 4&t=391848" onclick="window.open(this.href);return false;

For a carbine (which is what it sounds like you are probably going to build from the parts above), I'd stick with a 16" barrel. I personally prefer the lightweight profile. That would leave you with a choice of a mid-length (http://www.bravocompanyusa.com/BCM-16-M ... d-16lw.htm" onclick="window.open(this.href);return false;) or carbine gas system (http://www.bravocompanyusa.com/BCM-Stan ... r-16lw.htm" onclick="window.open(this.href);return false;). Either gas system would work for a general purpose rifle. From my limited understanding, the mid-length gas system puts less stress on the operating parts, but is not "standard" or milspec, like the carbine gas system (used on the M4 carbine).

Here is a thread that details assembly of an AR15 from parts: http://www.ar15.com/forums/t_3_4/226782 ... ions_.html" onclick="window.open(this.href);return false;

You don't need a special tool kit to assemble the lower, though a roll pin punch doesn't hurt. You will need an armorers wrench and a receiver block if you plan to assemble the upper.

My suggestion would be to buy an assembled upper. I am partial to BCM for uppers and bolt carriers (http://www.bravocompanyusa.com" onclick="window.open(this.href);return false;). Sign up for their email updates on out of stock products and eventually you will get one.

Do you mean should you get an upper with a 1:7 twist rate? The 1:7 twist rate is current milspec and will stabilize the heavier 5.56 or .223 bullets (77 grain sierra match kings, etc) as well as the 62 grain and 55 grain bullets. 1:9 twist is generally found on a lot of commercial rifles and will generally stabilize at least a 62 grain bullet (M855). 1:12 twist was found on the M16, M16A1, XM177 / CAR-15 series of rifles and will stabilize a 55 grain bullet (M193).