The BEST fighting shotgun accessories I know of are:
http://www.yfainc.com/courses.html#shotgun1
http://www.randycain.com/shotgun.htm
http://www.defense-training.com/courses/shotgun.html
... and so on.
Nope, you get no kewl points for having a neat-looking scary black shotgun, no kewl pictures to post on the internet in your MySpace account, no groovy Youtube videos to impress your friends.
You get to make YOU a better shooter. You learn how to FIGHT with a shotgun, at the hands of a world class instructor. You improve your own personal software in ways that nothing will ever take away from you.
And none of it has anything to do with hardware, either.
Louis Awerbuck suggests 4 things for a fighting shotgun (from my class notes):
1) A stock short enough for you to shoot comfortably. Most shotgun stocks are too long for many shooters.
2) A sling- slings on shotguns are as necessary as holsters for pistols.
3) A white light source mounted on the gun.
4) Sights, if you need them. Sights should be steel and firmly mounted, screws or silver soldered on.
That's it for hardware suggestions from the man I think of as the master of the fighting shotgun.
His own gun that he lugs around with him as he teaches classes all over the country? A cut-down SXS double barrel. And he only uses slugs, he says he isn't smart enough to keep up with more than one kind of ammo.
Per your questions:
Stick with the 870, to start with.
Your stock? Whack off an inch or an inch and a half if it's long for you (and if it's wood), and mount a premium recoil pad (Remington R3, LimbSaver, etc). Forget putting a carbine stock on a shotgun, unless you are one of those poor unfortunates who got taught to shoot by Uncle Sam and never shot anything but an M-16 based platform. Traditional shotgun stocks were developed to aid in speedy POINTING of the gun. Why fight hundreds of years of evolution if you don't have to?
The 18" smoothbore barrel is a good idea on a defensive gun. A touch of choke (most new ones these days come with ImpCyl as factory standard) is a help over a straight cylinder bore IMHO. I like MOD if I can get it but those are few and far between without resorting to choke tubes- but once in a coon's age, Remington makes some. Getting the forcing cone extended will help more consistently to improve buckshot patterns than most anything else. If you plan to use the gun for wingshooting too, keep any sights on the barrel so they go away when you take the barrel off.
SureFire fore-ends are too short to leave enough room for my gorilla-sized mitt on the forearm. And that big ol' hump for the light to screw into beats my hand up under recoil. I gave up on SF some years ago, and use rail-mounted lights now. The rails (from Streamlight) mount under the magazine cap, or on a magazine extension.
Magazine extensions can help or hurt, depending on how good an extension you get. Remington's factory job is best IMO. With an 18" barrel you can mount a 2-shot extension without sticking the extension out from under the muzzle. That should do. And no matter how much you spend on an extension, put a clamp on it to reinforce the junction with the threads at the end of the magazine tube.
One of the facts of life is that no shotgun magazine is ever big enough, and you're better off learning to feed that puppy on the run than struggling to squeeze on just one more round of magazine capacity. That's where those class suggestions come into play again, BTW 8^). Nothing will teach you coolness under fire, keeping your gun loaded and shooting and un-fumbled-with more than a good class under a good instructor. Except a bunch of serious gunfights that is, and your chances of living through the class and learning a lot are MUCH better than surviving a bunch of gunfights.
Have fun and stay safe,
lpl/nc