Jim, a suggestion....
Assuming your setting up a "Serious" shotgun, here's how I'd do it.
Start off with the stock, getting it fitting you and installing a decent pad, if none's there now.
While some like the full stocks with a pistol grip, I find the standard stock works well. And leave it a trifle short. Most folks do fine with one of factory proportions.
Then, before any other changes, shoot the thing extensively.Instead of adding the mag extension immediately, learn fast reloading techniques and use them under stress.
Once this is done, then add a 2 round extension. Longer extensions get mighty awkward, Bruce Buck's carrying a pig on a snow shovel analogy is apt.
And any crisis that is not resolved with 7 rounds or so of shotgun ammo is probably not going to be resolved with 10 either.
Put a short bbl on it with your choice of sights. A bead works fine.
And then make sure the trigger is clean, safe and around 4 lbs in pull weight.
Now shoot the thing until it feels like a body part.
After a few cases of ammo, you'll be in a better position to choose addons and mods.
HTH....