Welcome to MO Mike! I'm guessing you're in law enforcement? Thank you for your service to your community!
Modifying from factory can take on so many different forms its difficult to make suggestions without knowing how you want to purpose the shotgun. Will it be a personally owned duty firearm, something for competition, dual purpose for HD and maybe hunting or even a zombie gun?
There are some very practical upgrades that are almost universal if it will be purposed for HD, a shorter LOP stock like a Hogue stock and forend kit are nice, or even an adjustable AR or M4 type stock for familiarity from firearm to firearm though I don't personally like the ergonomics and mechanics of these stocks with the specific ergonomics of OFM shotguns.
Something else I recommend for an HD firearm is a light and a night sight bead, my choice is the Meprolight but you almost have to ask the manufacture date of the sight because the tritium has a limited shelf life and some have reported sights goin dim shortly after purchase, but id say this is not extremely common, just that it does happen and one should be aware of it if considering any of these sights.
Lights, so many to chose from. I like the CDM Gear mounts, though I don't think the one I have specifically is made anymore, kinda a bummer. He still makes some very good equipment that will fit in very little space and adds little weight, could even add a sling mount too, though slings aren't often high on the priority list for HD firearms.
I have a couple of Coast lights, each has been excellent, my only gripe would be they aren't made in the US. I've heard only good stuff about Elzetta and Streamlight, Surefire too if you have the coin. The Streamlight Polytac. Has a great reputation and for afair price as does the TLR series at a little more cost.
Big things in a light to look for or consider in a light would be momentary on and strobe should you want it. In a mount, my preference is to get the light as far forward as possible and still be able to operate it without my hand leaving the natural position on the forend.
If you were to use a railed forend of some sort, you could use a verticle foregrip with an activation switch built in, or a forend with a recess built in for a remote tape activation switch. Another more expensive option would be forends with lights built in, like the venerable Surefire forend or the newer Insight forend which OFM has been offering on some models.
So, lots to consider, narrow down for us what you need and we can get you some better info!