The best accessory for a new pumpgun is wear marks on the action bars. IOW, shoot it 'as is' till you get good with it, then see if you can find things that make it better at its job.
Starting points for a serious defensive shotgun IMO are a stock short enough to fit you properly, an on-board white light source, accomodations for a sling, and spare ammo on board the gun. Formal training for the shooter should come along in there somewhere too.
But if you just want to post pictures of your creation on the net and impress your friends with how bad your "shottie" looks, just order one of everything kewl in the catalog, bolt it on and don't worry about it.
Your choice...
lpl/nc