Under 16" you either have to apply to the ATF for a stamp for a short barrel rifle, or yes you can get a barrel with a permanently attached fake suppressor, but that's stupid. No point in sacrificing barrel length just to have something worthless still sticking off the end.
Most of those rifles you posted probably cost at least 2 grand, lots of fancy pants aftermarket parts there. And I promise you none of those are "lightweight"
What exactly are you wanting to do with this gun? You said something about for when zombies come, so it sounds like you want something you could actually fight with. If that's the case, you want it as light as possible. Flattop, with an Aimpoint or EOtech on top, skip the bipod, 16" barrel, front grip is personal preference, I don't see the point in them myself, get a decent sling, and don't worry about getting a free floating handguard.
If you want a long range sniper, well I'd say just get a .308 bolt gun. But if you are wanting to do it with this platform, then do pretty much the opposite of what I said above. Get a longer barrel and a better trigger.