I have a M1S90 that came with a 24 inch barrel and I just recently traded a 3 gun shooter for his 21 inch VR barrel. Seems he wanted my longer barrel to have a longer mag tube. Since I had been looking to trade for a 21 inch barrel for over 3 years, I JUMPED at the chance. Now I have to get a mag extension, a barrel clamp, maybe an enlarged safety button, and maybe a new bolt handle to make it look better than the stock one.
You'll find that the Benelli M1 and M2 shotguns are THE shotgun for the 3 gunners, as was mentioned earlier. They just go and go and go. Now to be honest a great many of these shooters have their stock M2's tricked out by these master shotgun smiths to squeeze ever ounce of performance they can get out of them. The sky is the limit on what you can spend too. A good friend of mine owns a bicycle shop and he has a sign hanging up that is appropriate here. The sign says "SPEED COSTS MONEY....HOW FAST DO YOU WANT TO GO?"
I am going to use mine as a HD shotgun and some playing around on the clay target ranges. Sure I might like to get into some 3 gun stuff, but I wouldn't know where to start or how to get into it.
If you plan on just doing recreational shooting at the range and maybe some HD use or occasional shooting games, you'd be just fine with just a few updates like a mag extension and maybe an updated fiber optic sight. Those are things only YOU can determine though after you shoot your shotgun and get familiar with it.
One word of warning though, when you start playing with the Benelli toys, be prepared to SPEND. NOTHING that comes from Benelli USA is inexpensive. A new barrel for a M1 or M2 is almost $500. Mag extensions are around $80 or so. The money can add up very quickly so make changes as your budget allows. USED stuff is also very difficult to find and when you do, it will also be expensive.
Of course all of this stuff is irrelevant if you don't plan on doing any customization or parts changing. If you are happy with your M1 as it is now, FANTASTIC. Get out and start shooting her. Pick up some different boxes of target ammo and see what works best for you. I'm certain you'll find some cheap ammo that will work like a dream in your M1. If you have problems, just ask for help. There are TONS of really knowledgeable Benelli fellas hanging out here and on other forums that would be more than willing to help with any problem you might run into.
Congrats and hope you enjoy your M1. How about posting a picture and range report when you get done?
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety"