Pump or auto?
Pump. Fewer parts to break.
Finish?
I prefer blued or parkerized, some prefer stainless.
pistol grip or rifle stock?
Stock. If you go with a PG, be advised that they're much harder to manage and require MUCH more practice.
sights or simple bead?
Bead.
mag size?
Most HD situations don't require more than 1 or 2 shots, but I say more is better in questions of capacity.
gauge?
Largest you can shoot quickly and accurately. 12- or 20-gauge.
other accessories (light, side saddle, etc)
Lights: I don't like them. They can give away your presence if you use them for movement as opposed to target identification. They're also one more thing to break. Sidesaddles or buttcuffs are good, but again, you need to practice loading from them.
brand?
The general consensus is Remington 870 or Mossberg 590.
slugs? OO? Birdshot?
Depends on the intended use and environment. Slugs in urban/suburban settings are bad juju. 1 Buck is my personal choice, but the walls of my cottage are very sturdy. Bird will turn a goblin into mush at typical HD distances.
most impt HOW MUCH SHOULD I EXPECT TO PAY FOR ONE I WON'T EVER HAVE TO REPLACE?
Don't base your purchase on the presumption that the gun will never break. They're machines, and they WILL break, eventually. I've heard of shotguns well over 100 years old that are still taken out and exercised regularly, but those guns have been well cared-for.
My personal preference is the Mossberg 500A, 18" cylinder bore with plain bead sight. No lights or slings, and a 6-shot buttcuff.
HTH.
