A lot of this depends on how much you want to spend, and what you really want.
The top of the line IS the Remington 870, in either an Express, or if you have the loot, a Police model.
For a look at what's commonly available in 870 defense guns, take a look at Remington's Law Enforcement site:
http://www.remingtonle.com
This will give you an idea of what you might like in sights and accessories.
Most of these accessories can be added later, often with little or no gunsmithing.
You can buy a police model by finding a dealer that buys from a Remington Law Enforcement wholesaler. Not all Remington wholesalers are LE sellers, so you may have to look around.
As to the pistol grip-only stock:
MOST people who try one quickly go back to a regular stock.
The pistol grip stock is a "Hollywood" thing that just doesn't work in the "real world".
The pistol grip is usually used only for specific special purposes like SWAT team door hinge blower, or body guard car gun.
In real shooting, the gun recoils badly, and is difficult to aim.
A simple, realistic shooting test will show you that a shotgun MUST be aimed with some precision, since you really can MISS.
They aren't like the Hollywood scenes where the gun blows an entire store window out, and still knocks the bad guy backward 10 feet.
The pistol grip shotgun is difficult to use, and offers no real advantage to most shooters.
I'd try a pistol grip equipped gun, then a stocked gun. If you're like most shooters, you'll put a standard stock on.
As to barrels:
Rifled barrels are only good for slugs. You're best bet is a standard cylinder or improved cylinder bore barrel. These are the most common and popular defense gun barrels.
With one of these, you can shoot any size birdshot, buckshot, or slug.