As far as BCG, you can get reasonably priced M16, Full Auto Rated, properly staked gas key. Not sure if you are set on any NiB or are fine with the "normal' phosphate BCG. Palmetto State has them for $90.
As far as handguard/rail goes, it is ALL preference. Quad rail, keymod, m-lok, tubular. Once you know what you like, you can start looking at manufacturers and narrow it down to your budget.
I can't help much with barrel since I only have 16" ARs and they are CHF chrome lined from Daniel Defense. I can say they are exceptional barrels though.
Flash Hider vs Brake/Comp is another thing you will want to research. If you can't decide then I would get just a standard A2 flash hider until you do figure it out and determine what best fits your shooting needs.
Sounds like that is all you are needing and are making great progress.
Edit: Charging handle I would look into BCM gunfighter. Not too expensive and highly regarded.
Buffer tube assembly and stock, I would see if palmetto state has a kit with the 7075 aluminum buffer tube. I know their complete lower build kits (LPK, buffer tube, stock, etc) is a great bargain bundle. But it looks like you already have the LPK, so no sense in buying another one.
Here is a link to a Stock Kit from PSA: http://palmettostatearmory.com/catalog/product/view/id/1145/
It doesn't have a fancy stock, but for $40 it is definitely a fair price, could even be considered a bargain price.
Edit2: If you are wanting a magpul MOE stock with the kit and like flat dark earth color, PSA has: http://palmettostatearmory.com/catalog/product/view/id/1898/
$30 more than the above link for the standard stock kit, but has a "fancier" stock.
Last edit: If you end up buying anything from PSA, make sure you get it all at once so you aren't paying a lot of shipping.
Don't forget for the upper you need an ejection port cover assembly ($6 from PSA0 and forward assist assembly ($13 from PSA).
I think PSA has great "bang for your buck" components and use them for a lot of smaller parts on some builds. My higher end/customized/no expense spared builds get a little bit different things from other companies that I pay more for, but it has nothing to do with function and more in favor of adding flair to the rifle to make it unique.