I like the 870, OK I'll admit it, I love it.
The 20 Guage is fine for home defense, and if I didn't already have my fair share of 12 Guage guns it would be my first choice. As far as what modifications, no longer than a 20" barrel, tritium bead, spare ammo carrier of some sort and an illumination device would be what I'd select.
18" or 20" Barrel is preferred, IME. Different companies make a tritium front bead that can easily be installed by a Gunsmith. I'm using Ghost Rings on my gun, but that's because I also shoot out to 100 Yards with my shotgun. That's not realistic for a home defense gun. BTW, after 35 Yards I'm resorting to slugs.
As for spare ammo, while many people like a side saddle, I prefer my ammo carried off the gun to keep the gun lighter so that it swings better. I have a pouch that holds my shotgun shells, and I drape that over my shoulder when I pick up the shotgun.
You don't need a Surefire Fore-End, IMO, but you do need to mount a light of some sort on the gun with a pressure pad. It makes identifying the target much easier and the pressure pad allows you to turn the light on/off with hand pressure. Lyman makes a decent one for around $80 that can be mounted at home easily.
I like extended mag tubes, but then I have a known habit of carrying lots and lots of ammo. Shotguns, like six shooters are known to lack an ammo reserve of what many consider sufficient quantity. Hence, the bag over my shoulder with fifty more rounds. I do not know if they make an extended mag tube for the 20 Guage, but then I haven't looked.
Biker