Just saw this thread now. If you are wanting to build a solid performer, I'd recommend the following.
For barrels, you can't really go wrong wit Daniel Defense. If you want an SBR, get an 11.5" it's the best option ballistically and best option for reliability in an SBR package. If you want a more traditional carbine, I'd say grab a DD 14.5" lightweight barrel (mid-length). Don't listen to anyone saying that LW barrels are bad. They are absolutely fine for general running and gunning.
Pick any receiver that suits you. The forged options are the best priced, and you can get some cool Spikes Tactical ones. But there are some rally slick billet receivers out the. The Rainier arms MOD3 ones are slick and ambi.
For BCG, grab either a DD, Spikes or Rainier Arms. These are VERY well known brands that will serve you well. Nickle boron finish is nice but definitely not necessary unless you want to have a slightly easIer cleanup after a longer range day.
If you're already familiar with ARs, get a Geissele SSA trigger. If the price is too much get a G2S trigger.
If you are not familiar with the AR platform, just get a complete DD lower parks kit or a Spikes Tactical Enhanced lower parts kit (the spikes also comes with a nice pistol grip too).
For a rail, I personally think MLOK is the way to go, but your milage may vary. Midwest Industries makes EVERY rail you could possibly want. They are well priced, well made and lightweight. Perfect for any build.
Everything else is personal preference. I like PWS muzzle breaks, buffer tubes and buffers. But you could really go with any generic parts for that.
Stocks and sights would be personal preference, but Magpul makes great stocks and BUS. For a RDS, it's pretty hard to beat the Trijicon MRO for price, quality, durability and features. I'd pick that up over an Aimpoint micro now.
If you want a 1-4 or even a 1-6x, look at the Vortex PST or Strike Eagle respectively.
Keep us all updated and I hope this helps!