Building vs buying means understanding that you could do either more cheaply. It really depends on what you intend it to do. For the use described, a $599 Bargain Bin gun would get it done. It may not seem like it offers the reliability you want, tho.
Problem is, the gun is already being built backwards. Choosing the furniture before the caliber is getting the cart before the horse. Taking the concept of self defense at the end of the world, look at what has already happened in disasters or uncertain economic times. Military surplus dried up, that ammo and guns became scarce, and in some localities, authorities went out of their way to confiscate firearms.
I found .30-30, .30-06, and 12 gauge on the shelf throughout the entire "Obamascare." 5.56, .308, and 9mm was non-existent. So, for actual use, avoid the military calibers. It will be hard to get ammo from those sources. And the ridiculous notion it will be laying around is BS. Soldiers don't leave their buddies behind, and they cross load the ammo to resupply their needs. Armories have none, nada, zip. If YOU don't have sufficient stock and stay put to protect it, wandering about means whatever caliber you have on you is about all you'll need.
As for an AR, the better plan is to build it knowing what you will actually do with it all the time life stays normal. That's what you'll be doing anyway, practicing to maintain skills and rotate the ammo stock. Consider reloading, as that means you will be saving money.
Pick the caliber - AR's offer more than 5.56, much much more, then that caliber and use will determine what barrel length, which sets the gas length. Then what type upper, likely a railed A3, to support the optic of choice, likely a red dot. THEN furniture. Standard mil issue A1 or 2 buttstock will do, size a grip to fit - and avoid the multipiece ones, as all the little pieces do nothing to improve accuracy. An A1 or A2 grip is all you need and dirt cheap. Handguards are all you need - accuracy comes from a good barrel, and milspec ammo is only good for 2MOA, which is all that is needed. A free float won't add accuracy, it just keeps the sling from pulling the point of impact around. If you're really scrambling, a sling will hang you up, and the Infantry School had us delete it in the field. Try that and see for yourself - combat use of the rifle isn't a three gun range competition. Murphy and the worst case happen every minute.
What you will have as a result is going to be a lot more like what the Army issues for real combat, not a fantasy gun build. And that is still the M16A2 pattern - or A4 in the Marines. About all that gets added might be a Norgon ambi release. Most of the soldiers using the M4 keep the quad rail empty to avoid dead weight. Even KAC has said it's not needed for civilian use. The .Gov adopted it because too many different users and gear were involved, it's an institutional compromise. Consider: it does nothing more than act as a light mount - a very expensive $150-300 light mount. That is all.
Start from square one, pick the caliber and barrel length, then move to the rest of the gun in sequence, and it becomes something you can actually use well for your intended purposes. If there is anything to note about the choice of Magpul, it has to be asked, why aren't PMags the first choice? They have far more to offer in reliable function than any of the furniture options already listed. It makes it seem the direction of the guns is for looks more than actual shooting.
