Contrary to net Myth, a bone stock pump shotgun right out of the box is an awesome weapon that does not limit the shooter. The shooter limits the shotgun.
Best advice I can give is to obtain a US made pump like an 870 or 500 and shoot the bejabbers out of it.
When the empty hulls figuratively reach your shins, and you've installed a nice set of wear marks on your shotgun, then you'll have a better idea of what you and your weapon need and what's just shotgun bling.
Don't get me wrong, my HD 870s have "stuff" on them, but they had a good round count before I reached for the catalogs or phone.
And, some accessories are counter productive. I've never seen nor used an adjustable stock that was as comfortable as the wooden version. If a shotgun makes someone uncomfortable, they shoot less and that's not good.
So, what you need to buy now is ammo.