Some thoughts on HD and shotguns from a retired Firearms Instructor.
Shotguns only work in a HD scenario IF ALL adults in the house are trained and effective with same.My wife dislikes shotgun but shoots handguns OK, so the Tactical 870 is my weapon, not hers.
As for those pistol grips, they're fine if you're auditioning for Miami Vice II,but not in AS scenarios. When I started instructing for the Md Dept of Public Safety,we had folding stocks with pistol grips for some rapid response vehicles. The Troops had a much harder time qualifying with them.
I'd only use hip shooting at contact distances. If you insist on so doing, practice much. Keep your forward elbow locked, or you'll hit high/right.
As for mag extensions, I use them more for recoil control than extra ammo. Face it, if a scenario takes more than 4 rounds of 00 to resolve, you probably need backup,not extra shots.
Also, if you can find a gunshop where police depts turn in old inventory,you can save a bundle.One of the 870s here at Casa McC started life as a cruiser weapon in Arlington Va, another in a gun tower at the Md Pen.Both will outlast me by several generations.
As for the bells and whistles,the best accessory is a coupla cases of ammo and some range time.
Hope this helps...