Here's some input that I learned from setting up and running my HD setup.
Vertical foregrips on a shotgun are 100% Hollywood. In real life they cause you to put a lot of downward pressure on the front of the gun as your cycle the action. This means you are pulling the muzzle down and off target every time you rack the slide. That's a bad thing. They also give you less lateral stability. You tend to wag the gun from side to side as you pump the action. That's also bad. You want as much of your support hand as possible wrapped directly around the forend of your shotgun. This gives you the most stable grip and keeps the muzzle pointed at your target as you pump the slide.
Also, VFGs are known for tearing out of the M-Lok slots on the Magpul MOE shotgun forend, due to the mechanical leverage applied when racking the action on a shotgun. Magpul even recommends against using VFGs on the MOE shotgun forend. Remember, your shotgun should always be pumped with authority. Don't baby it. Rack that slide hard! You can't over-do it. Slow and wimpy racking leads to malfunctions.
A sling is a good idea for weapon retention (hopefully a bad guy never gets close enough to grab you shotgun!) and for times when yo need you hands for purposes other than holding your shotgun. A 2-point sling is superior for both of these. Yes, a single point sling looks more "tactical" and is more comfortable if you haven't learned how to use a 2-point sling. You also need to practice using your sling. (FWIW, a long gun dangling from a single point sling will find your groin 99% of the time!
Get out to the range and shoot your gun IMMEDIATELY after making each modification. Things that feel great while you're plating air guitar at home might be terrible when you actually shoot the gun. My first light setup was an InForce WML on the support side of a Magpul MOE foregrip. It looked great, and my thumb fell naturally on the activation button, so it was easy to turn on and off. It was great when practicing at home. Then I shot the gun... The light POUNDED the tip of my left thumb with every shot. After a few magazines I had to take it off the gun. The exact same position of the same model light is great on my AK, but awful on my 12 gauge. But that didn't come to light (sorry for the bad pun
) until I put some rounds though it.
Speaking of lights... I feel one is a MUST for an HD shotgun. You need to be able to identify your target and what is behind it. The last thing you want is to shoot the family pet or a family member. I just ordered the TL-Racker forend from Streamlight. It looks ideal to me, but I'll weigh back in after I get to use it a bit.
On that note, be willing to undo a modification if it doesn't turn out to work as well as you hoped. It sucks to pay good money for something that you end up not using, but that's better than stubbornly using something that doesn't work well. I have a box of failed accessory attempts from various guns. Who knows, what didn't work on one gun might be perfect on a different gun in the future.
Sighting is important! You can just point in the general direction of a threat like they show in the movies. At minimum, I feel a HD shotgun should have bead that is visible in the dark. That means installing either a fiber optic or Tritium bead. My preference is the tritium Big Dot from XS Sights. I'm not a fan of rifle sights on HD shotguns. Yes, the look uber-tactical! But they are better, IMO, for longer range work than will occur in a HD situation. The police may have to shoot out to 40-50 yards, but most HD scenarios are inside of 10-20 yards. I really like red dot sights for HD shotguns. I have a Holosun circle dot sight mounted on the receiver of my Mossberg. Target acquisition is super fast with both eyes open so I can see whats going on around me. It also makes getting a perfect cheek weld and eye alignment much less critical.
Stock length... go as short as possible. This is completely different from fitting a stock for wing shooting.Yes, it will feel cramped when your facing forward, but this is a dedicated HD gun, not your waterfowl shotgun. A proper tactical shotgun stance is more squared up than when you are shooting clays. A shorter stock will feel longer when you square your shoulders to the target. You want to be able to point the gun as far as possible to your strong side and still be able to work the action. If your support arm is stretched too far, you won't be able to do this well.
Finally, practice, practice, practice. All of this is academic and flies out the window if you can't put it into action when it counts. HD and bird hunting are two completely different disciplines. Shooting clays does NOT prepare you for man sized targets inside a house at 7 yards! Practice HD scenarios with your HD gun and HD loads. Don't practice wingshooting. (That's like practicing parallel parking and saying you are prepared to race autocross. Yeah they both use cars, but they aren't the same thing.) Getting an hour or two of professional instruction will yield huge dividends in effectiveness and confidence.
I hope you find something in that long, rambling diatribe that you find useful. As always: your mileage may vary, all of my opinions are worth exactly what you paid for them, I'm not a gun instructor nor did I stay in a Holiday Inn Express, if redness and itching occurs, see your doctor.