to add on the barrel discussion, unless you are shooting over 800 yards, you won't see any real improvement from a 20" match barrel to 24. more or less as vague rule of thumb a 16 is good to 600 yards, 20 to 800, and 24 to 1000+. You may want to consider these guys on the barrel as well, you can custom order a barrel how you want it, which is nice
http://www.blackholeweaponry.com/catalog/index.php?cPath=48&osCsid=809c68469d97cf3467447f6c0b1cfc04
you are probably better off trying to source a complete BCG right now vs parts, particularly if you think you may switch to the jp bolt later. there is one place that has a preorder open
http://redrockord.com/shop/rro-ar-10-bolt-carrier-group-bcg/
I have no idea what the company or the quality of their products is like though, so i'm not endorsing them, just sharing what someone else posted here.
that handguard looks nice, but as has been mentioned you may be at the whim of what you can actually get vs what you want, unless you are willing to wait an unlimited amount of time. I'm building my first AR 308 (LR style) and started basically Jan 1st. I won't have all the parts till the end of April, and I as much as possible went with "what I can get", with just a couple of "what I really want" thrown in.