If you want the "ultimate" barrel, get one of the new FN 20" CHF GI profile barrels. Of course this is also the most expensive option, as not only will you have to pay for the barrel which is pricey, but will also have to buy a non F FSB and pay to have it installed.
WW barrel assembly would be the cheapest option available, should also be a good quality barrel.
Model 1 sales will be the cheapest place to buy a new A2 upper already assembled. Will also have the proper GI profile barrel that is chrome lined and 1-7 twist (though unfortunately, I've heard they now use F marked FSBs).
If you don't care about total authenticity, Del-Ton offers a 20" light weight barrel that is 1-9 twist and has an option for chrome lining. This will be a better general purpose field grade barrel than the typical GI profile that is heavy contoured in the front.
Can buy the complete upper from Del Ton or just the entire barrel assembly, which includes everything you need to attach to an upper (including handgaurds, front sight post, gas tube, etc). Can select a non F FSB too.
The chrome lining feature isn't selectable on the site, but if you request it in the comments section they will give it to you. I ordered one this way myself.
Del Ton's complete barrel assembly will be even cheaper than WW's. Of course, it won't be as "authentic" but I actually view it as superior.