if you're looking for iron sights i would suggest trijicon or scattergun technology ghost ring sights. XS makes good stuff too. you'll need a gunsmith to install them. i would suggest going to T&T gunnery in seaford to do it. that place in islip wouldn't do it for me because it was to difficult. if your looking for some sort of optic i would go with any quality reflex sight. Aimpoint, EOTECH, trijicon. these optics will cost more than your shotgun but there is nothing wrong with that. if you do go with a reflex sight take time and shop for a quality mounting rail. if your mounting hardware sucks your optic is useless. mesa tactical makes quality picatinny rails. also, get rid of your pistol grip and install a stock. it doesn't have to be fancy but if you want somthing high speed, again mesa tacitcal makes good stock kits. a factory mossberg stock will still do the same job however. just get rid of that pistol grip. pistol grips on shotguns seriously limit the capabilities of you shotgun.
YES you do have to aim a shotgun to hit what you intend to hit. it is not like in the movies. also, what kind of ammo do you intend to use defensively?