For defense, 16" is the best, and it will also work okay for three-gun. For varmints, I'd choose a 24" heavy barrel. For formal CMP/NRA type stuff, the 20" is required. A 20" H-BAR might be the best compromise if you'll only have one upper.I will be using it MOSTLY for target shooting, with a defense and/or hunting application. Does not have to be 20" long... I own an 870...
Tons. There are several tiers of ARs, though the lines get blurry. Colt and LMT are top tier examples. One notch down might be Bushmaster, Armalite, Stag, and S&W. In the lower tier are DPMS and Olympic.
1) What brands are established?
There are over a dozen AR makers, and even some of the lower tier guns can give good service. I used to own a DPMS that ran perfectly, for example.
You can build one yourself for $600 or so, according to the internet. Few guns built by amateurs seem to run perfectly under hard use, at least in my experience. But some do.2) Price ranges?
As in pistols, you can go all the way up to semi-custom stuff like Baers and Wilsons, for over $2000. Colts are around $1500, I think. The Stag runs around $800.
"Must have?" A sling and a bunch of magazines. That's all.3) Must-have add-ons (optics)?
But it's really mission-specific. A hunting gun calls for optics different from a fighting rifle. I know more about fighting rifles than hunting guns, so I'll just say that for defense, I'd want a red dot sight like an Aimpoint or EOTech.
I like the collapsible because it can be adjusted for shooters of varying size, different clothes, body armor, etc. I actually collapse the stock completely when shooting from the squared-up fighting stance. However, it is not as stiff as a solid stock, and hence slightly more difficult to shoot at distance.4) Colapsable or fixed? (I'm in FL where we can own full-auto/suppressed if we want)
You'll need a railed forend. There are many good ones. I have experience with the KAC and Surefire. The KAC is super robust.5) I like toys... so I want to add fun stuff later (lights, lasers, grenade launchers... etc.)