It's good that you stated your intended role for the gun in your post.
Because your purpose is for plinking out to 100 yards, but also self defense (possibly in the confines of the home), prevailing wisdom (and my personal experience) is that an M4 barrel configuration is the better way to go. It offers a slightly lighter rifle, but the shorter barrel makes it easier to handle around corners in the home. Accuracy difference out to 100 yards between the 14.5/7 barrel and a 16" barrel is nil, and there's no appreciable difference in terminal ballistics either.
I agree with the post that suggests a .556 chamber over a .223, but more for the reason of flexibility in ammo choice, more so than velocity. Another consideration is barrel twist - I recommend a 1:7 or 1:9 so you can stabilize heavier bullets and likewise choose from a wider variety of ammunition.
If this is your first AR, I would suggest you buy a pre-built upper, and only tackle the (easier) lower, as it doesn't require special tools other than what you probably already have at home. The upper build requires special tools (clamp/vise, wrench, etc.) which would mean that whatever you save by building your own is going to be spent on tools, unless you happen to know someone in the neighborhood who already has them. There's a lower receiver
builder's guide at AR15.com which you'll probably find very useful. It's what I used when putting mine together.
My standard paradigm when making this sort of decision is "buy it once, buy it right", or stated another way, "buy the best you can afford". You have a real good choice there with the PSA or Colt, and though it might be a little more "expensive" now, the temporary pleasure of a lower price is far outweighed by the long term pain of lower quality. By the way, I put the word expensive in quotes because it's a relative term - just because someone sells something for half of what the market value is doesn't mean that the other choices are overpriced.
Even if you have to spend a bit more now, remember that one of the really great things about the AR platform is that if you ever decide that you wish to change the configuration, you can simply buy a different upper, and do so WITHOUT A NICS CHECK, since only the lower receiver is tracked by ATF, unless your state's laws vary, of course. Just bolt it on and go.
You might also look at these makers, who are popular with the AR15.com crowd:
Rock River
CMMG
DPMS
Armalite
Hope this helps!