I am a legal NFA owner.
What you are doing is called building a SBR - short barreled rifle. Before you buy the new barrel - you need to fill out a NFA Form 1uising the serial number of a rifle you already own, have your chief of police sign it, and include your photo, fingerprint cards, and check to $200 and mail it off the NFA Branch. Takes about 3-5 months for the paperwork to process.
Once the tax stamped Form 1 is returned to you - now you can add the barrel and make it what ever length your heart desires. Most people use the 11.5" barrel in conjunction with a 6" suppressor - the short barrel is a huge help when making a subsonic round that cycles the AR, and, if you have ever used a suppressor on a 16" barrel you know how bad it sticks out.
Once you have a lower designated as a SBR you can never sell it - not without doing a transfer through a Class 3 dealer. You can have it transfered for free to a relative after you die (on a Form 1). For this reason - pick a nice top of the line lower. There are some companies that make really sweet custom AR lowers that look allot different than a normal one - they cost around $320 each - but if you are gonna be stuck with this one for the rest of your life - might as well pimp it out.
Hope this helps