I woke up to 400 black listed IPs so I'm kinda busy today. This is more or less just random thoughts, I'll try to reformat it later
Decide if you want a fixed front sight or if you want a free float rail. Personally I like the fixed front sights, a free float rail build will cost $200-500 extra when it's all said and done. I also think that quad rails are a waste for most people
The main difference between the standard 16" AR barrel profiles is the weight. From lightest to heaviest: light weight, Gov't profile, M4 Profile. I like the light weight barrels.
Carbine and Mid Length is referring to the gas system, there's not a huge difference but Mid length will have longer handguards, a longer sight radius, slightly heavier and softer recoil compared to the carbine. I prefer mid's but there's absolutely nothing wrong with the carbine length gas system.
16" is the minimum legal barrel length, you can buy a 14.5" barrel and permanently attach (pin + silver solder) a 1.5" flash hider to make it 16". This can limit things later down the road - you will never be able to attach a different flash hider and most surpressors won't work with the 1.5" barrel extensions. It will also add to the cost and you'll only save maybe 1" so I would suggest sticking with the 16".
If you're buying a stripped lower, stick with a basic FORGED lower. I'd only suggest purchasing a billet lower if it comes matched with a billet upper.