Hi Quii,
I'll take a stab at it with my $.02. I'm not sure if you are planning to get a 16" or 20", but:
1.) Bull barrels (or HBAR's, if that's what you meant) are heavier, but don't heat up as fast as "thinner" profile barrels. They will help offset muzzle climb. Though, others will tell you that you will regret it if your intent is "tactical work" or hauling it around all day. In that case, I'd agree and go with a standard weight barrel or M4 (my personal preference is HBAR's).
2.) Fluting will shave some of the weight off, but I, personally, don't care for it, except for under-the-handguards. Under the handguards would be sufficient to help with weight and added cooling. Full-length fluting messes up the smooth barrel lines and makes more sharp edges for the finish to wear off of (only my personal preference boys). Kurt's Kustom Firearms does the under-the-handguards fluting for $50.00.
3.)Muzzle brakes...they are loud. And ugly for that matter. AR-15's just don't have much, if any, recoil. So, a muzzle brake is a waste of time and money. The weight of the HBAR barrel will help with the muzzle climb. If you want something on the end of a post-ban barrel to give it a "pre-ban or finished look", check out the fake flash hiders on Kurt's site.
4.) No experience with DPMS rifles, but I'd recommend Bushmaster. They have good prices and excellent quality. They also have the barrel fluting (full-length only) feature for $50.00 and a selection of muzzle brakes (wouldn't recommend them though). Also check out Rock River Arms. I hear they have some nice, quality items too.
Hope this helps. Good luck with your project.
john3:16
Here's the links to those sites:
www.bushmaster.comwww.kurtskustomfirearms.citymax.com/page/page/8932.htmwww.rockriverarms.com