The remington 870 is considered the most reliable shotgun ever made. The mossburg 500 is right up there also and preferred by many due to the location of the safety on top of the receiver as opposed to the870's by the trigger like a BB gun. I would recommend a pump over semi auto for self defense. less moving parts, less room for malfunction, less expensive.
Stainless is also unnecessary. Glean the gun after you shoot it and it will not tarnish or rust.
The main thing is to get the gun and shoot it often. Be familiar with the weapon you are protecting yourself with.
they do kick rediculously hard especially high brass buck shot which is what you'd want for HD. a good aftermarket butt pad helps quite a bit and knoxx makes a recoil reducing pistol grip stock ive heard good things about.