I'm assuming you want the 870 for HD. Shame to limit it to that, the piece's one great all round shotgun.
Been shooting 870s since roughly 1958. There's 4 here at Casa McC, now, of various configurations. Built a couple of them at the kitchen table. Here's a few suggestions.
First, the stuff best left to a qualified smith. While the 870s usually come with an acceptable slap trigger, the best work can be done,especially with slugs, with a trigger worked down to maybe 3-4 lbs.
While the trigger's getting tweaked,have the smith lengthen the forcing cone. This keeps pellets in the pattern and slightly reduces recoil.
Next, if you are barrelchested and/or the weapon will be fired by a woman also, use some sandpaper to slightly round off the toe of the recoil pad.This stops it from digging into soft tissues and reduces the chances of getting the flinches...
As for the accessories, don't be in a hurry. The next thing to buy is lots of ammo and some range time. Shoot some trap,skeet or sporting clays to work the weapon in and improve your handling of same.
As for extended mags,I like them for reasons other than mo' shots before reloading.
Face it, any A-S scenario that can't be resolved with 4 rounds probably needs backup rather than two more rounds in the tube.
But, the extra weight of the extended tube helps recoil control, and clamping the tube to the bbl oft shrinks slug groups dramatically,effectively stiffening the bbl.
A caveat, some folks think a shotgun can substitute for shooting skills. Inside 25 yards, the shotgun is perhaps the most effective available, if the shooter knows what he/she's doing. If other folks may need to use the weapon, ensure they know what they're doing,and provide alternatives for a spouse or grown child that doesn't like shotguns.
Finally, I've shot hundreds of rounds from various positions. At more than contact distances, I'll shoulder the weapon. Pistol grips are egregious, stay with a full stock.
Hope this addresses your concerns...