First, what is the goal?
Like putting 24" low profile tires on a 4x4 pickup, anything can be done with enough money.
Now, why do you want to spend your hard-earned dollars on stuff for your AR-15? Railed forearms look neat but, you aren't attaching night vision devices or thermal scopes either. A forward grip looks cool but, unless you are a serious 'door kicker', probably not a good reason to add a railed forearm.
Then there is the weight issue. I don't care if it is a Remington 870 with an extended magazine or an AR-15 with a bunch of lights and other stuff, can you really hold a firearm with that much weight in shooting position for any length of time? Weight at the end of the muzzle is harder to hold up, gets very heavy over time, and totally kills dynamic movement or tracking a moving target (unless you can track Geese with a 32" shotgun).
For 'urban defense' in a SHTF scenario or just general plinking, you don't require nor need to pay for the training to hit 'man sized' targets at 600 yards or more. Sure there are AR-15's and shooters that can do that reliably but, unless you spend a good deal of time training in the military this is unlikely to be a realistic goal for most civilians. Optics help but, you still need an accurate rifle and ammo and a trigger puller that has the skills to make the shot.
The 7.62x39 round is a very good round for 'urban' shooting distances, targets, and barriers in a SHTF scenario. Sure a 308W is better but, it is also heavier and could be problematic with over penetration in your suggested use scenario. The AK-47 semi-auto clones and yes, SKS, are good rifles for this type of use and general plinking fun. You won't light matches or shoot flies at 50 yards with them but, they are generally very reliable and easy to maintain. Adding optics can be done but, is a bit problematic unless you spent big dollars for a lot of mods. The optics attachment on the side of the AK receiver works okay but, it isn't ideal either (long arm, high bore sight, etc.).
A bolt action rifle might be a better choice for you. A Remington 700PSS in 5.56, 308W, ... is a great platform for optics and longer range shots. It may not be mall ninja cool but, a solid bolt action in a mid-range caliber is a great compliment to a serviceable AR-15. Personally, I'm a fan of a Marlin 44Mag lever action rifle. Why? It is low profile and doesn't scare 'sheeple' near as much, draws less attention to me, and 240gr .44" chunks of lead pack a significant wallop at any distance most people could argue was "defensive" use.
Finally, for the money most people put into accessorizing their AR-15, they could get a sound suppressor (assuming you live in a free state) and cut the noise. The last thing I want in a SHTF scenario, is announcing my presence to every militia member, crack pot, criminal, gang banger, etc. that is around. Or in my case, shooting feral hogs at night with a thermal, upsetting a PTSD neighbor (thank you for your service).
