Though the problem is reported most by users of Mossberg 500s - I suspect because so many are commonly kept loaded in closets, under beds etc - most any tube fed shotgun can - and likely will - deform the shotgun shells left loaded in a magazine over time. I've noticed it myself with Mossbergs. Some people report this causing jams or other problems.
I've left Mossberg 500s loaded for literally years without the mag spring dying or seeming to have other problems, but the shells will deform.
Many shotguns from Mossberg, Remington et al have spare bbls available of a shorter length. This allows those who want to modify a hunting gun to do so with ease. Mossberg even makes bbls for the 870s and sells them as accessories.
You might want a light. Though a good (dedicated) weapons light can cost what a basic shotgun does, depending on what sort you get.
I've always liked hi brass, non magnum, OO buck. I'm not that recoil sensitive so I've never bothered with the various LE/tactical offerings, many of which tout reduced recoil (and reduced velocity). I actually liked Winchester OOO buck best of all, but it is hard to find. (The Ramones actually sang about it, and the 590, in a song "Ole Scattergun". Joey liked guns.)
A simple slip on shell carrier from Walmart (et al) can carry five extra rounds on the stock. These are best secured with higher grade duct tape or Gorilla tape. The shell holders - elastic - also stretch and wear over time. More expensive leather models can be had, or you can get a polymer side saddle.
The HR/NEF/what not Pardners are made in China.
Store brand pumps of American make can be had in many states for 150 dollars or less.
While a hacksaw can shorten a bbl (and open up the choke nicely) on a shotgun, it helps to crown the bbl (after it is cut) and get a new bead installed. I've got an old Savage Stevens of that type in the closet.
In "Free states" (those other than NY, NJ, CA, HI, DC and perhaps a few others) many people who once opted for pumps have come to prefer modified Saiga shotguns with rapidly loading mags (or drums). A cottage industry exists to sell these. Tromix Lead Delivery systems is one such maker. (Assuming that they didn't just get an M4 clone, which is the other thing people commonly get instead of a shotgun... but that's another story.)
Newer, purpose built, "tactical" guns will often have a dual purpose muzzle brake/breaching attachment on the muzzle. These can also be used as impact/strike weapons.
Pistol grip shotguns.... well they can be had in either the M16 type of "pistol grip" stock or the "just a pistol grip" style. Many shotguns can now be had with M4 style stocks, allowing the lenth of pull to be adjusted. If you're used to an AR, some of them fit naturally. If you're used to a traditional shotgun, you might not like them. M16 style grips are now sometimes seen on sporting shotguns and are said to reduce recoils.
There are other custom options in recoil reducing stock add ons, and some people get their guns ported (Vang comp, etc).
Newer guns may also have rails on top to mount an Eotech, Aimpoint, etc. Others can have a rail added by the same means one might drill and tap - if needed - and mount a traditional scope mount.
However you store extra ammo, it might be a good idea to have a few slugs handy. I've always like Brenneke slugs, other people just use good old Foster type deer slugs. I've never bothered with the tactical type offerings. It's hard to go wrong with a .729 caliber/1 ounce hunk of lead.
For buckshot, I tend to just but the 15rd value packs of OO (either Remington or Winchester) from Walmart. Saves a bit versus buying 5 packs.
Anyway... you can spend a lot of money and get a Benelli, tricked out Remington auto, etc... or you can just use an old store brand gun, Mossberg from Walmart, etc.
I keep two Mossberg 500s (they were cheap) and the Savage/Springfield/Stevens pump - all 12 gauges. I don't have optics or lights on mine, but I keep such on the carbines and rifles that I'd actually use. The shotguns are just... eh sort of there... and handy to have around.
If someone is even poorer, the old Mossberg bolt action guns are quite cheap when found. They are 2+1 guns and surprisingly quick if you are used to handling a rifle. I have one in 20 ga myself as a simple utility gun, in a pinch... eh, it'd do.
Oh and some Mossberg 590s will accept a bayonet. Just in case you are a pessimist. Or like options. (But the Marinecoat - or whatever they called it- 590 that I had years ago never would feed correctly.)
I bought buckshot yesterday, never heard of a ban. 
