The main thing to acquire is the skill set necessary to run your shotgun as a weapon rather than a sporting implement. Fighting with a shotgun is different in some fundamental ways from using it dusting clays or bagging small game. It's a different program for that fire control computer between your ears to have to run- be sure you install it well.
Training is a good idea, and you might not have to look far to get started. The basic NRA classes in personal protection in the home are pretty widely available- see
http://www.nrahq.org/education/training/basictraining.asp . If you've never had any formal firearms training at all, a Hunter Safety Course or your local equivalent is a good way to break the ice. In short, any GOOD training is worth your while- look around and see what you can find.
There are a handful of good books/videos out there that can help you get started. Anything from Louis Awerbuck is good, Steve Moses' book
Carbine and Shotgun Speed Shooting, Jim Crews'
Some Of The Answer, Urban Shotgun- An Advanced Shotgun Technique and Tactics Manual, John Farnam's
The Farnam Method of Defensive Shotgun and Rifle Shooting will all stand you in good stead. There are other similar works available as well, but IMHO nothing compares to taking a class from a first rate instructor. It takes time, and it's expensive, but it gets you further down the road to building genuine skill faster than anything else I know.
Find yourself a shorter barrel- 18- 21" or so will do. Bead or rifle sights or even vent rib (if you find a 21" Turkey barrel) will all work fine. There are plenty of factory made short barrels for 870s around, shouldn't be hard to find a good used one at a reasonable price. Last time I looked Remington was selling them new for about $100. Do make sure it's a smoothbore- there are some rifled ones around out there, and they won't do well with buckshot. Be sure you get a barrel with a magazine cap retention system that matches your gun too- either a detent in the barrel ring for older Expresses or the detent as part of the plastic magazine spring retainer as in the newer Expresses.
Make sure your shotgun fits you properly. If more than one person will be using the gun, it should fit the smallest person using it- it's easier to use a gun that's stocked a bit too short than one that's a bit too long. A premium recoil pad costs about $25, a bit more if you have to pay for installation, and is well worth it.
I like having a white light source on board the gun. The easiest option IMHO is to install a Streamlight #69906 light mount (about $13)under the magazine cap, and put any one of several pistol type lights on the rail. My most recent weapon light purchase is an Insight SSL-1 LED (about $75) and it's working well so far. YMMV on having a light on the gun of course, but if I have to grab a shotgun at 0dark30 I don't want to have to fumble for anything else.
I also like having extra ammo on board the gun as well. I like Sidesaddles, of course YMMV there too. Some folks like butt cuffs, others like SpeedFeed stocks with magazines built in. But I want extra ammo on the gun- again, one less thing to scramble for. Here we load magazines with 00 buck, and Sidesaddles with Brenneke KO slugs so it's easy to tell what's where and select whatever round fits the circumstances.
All the 'house guns' here have provisions for slings/carrying straps, since they double as Homeland Security guns also. Usually the slings aren't mounted when the gun is on duty inside. Some people (like my favorite shotgun instructor Louis Awerbuck) insist that a fighting shotgun have a sling at all times, that a sling on a shotgun is like a holster for a pistol. True enough, but a sling indoors is a pain in the tuckus and we do without them. YMMV there too, of course.
Basically the shooter matters a lot more than the gun, as I like to put it, 'software trumps hardware.' It's building your skill set that counts far more than building
THE ULTIMATE TACTICAL SHOTGUN!!!. As Dave puts it, BA/UU/R. Training and practice, in other words.
And Stay Safe,
lpl/nc