I'm rarely in this forum (tactical shotguns) but I take a peek in here once in a while. I am sincerely puzzled by the thread starter, and the OP's goals. Build-up an 870 Express with a ton of add-ons then not shoot it? Yes, it would be a somewhat intimidating-looking wall hanger, just a massive departure from how I see/use shotguns myself. Were I being asked for advice on the matter, I'd say slow down...strip the gun down to the essentials...shoot it a WHOLE lot...then decide what add-ons make the most sense. Not carrying? Forget the sling. Not shooting slugs? Lose the optics. No likelihood of being in a gun battle? Lose the shell carrier.
Goes without saying, but I'll say it anyway~in all but a few cases/applications (slugs+hunting) you kind of point shotguns..not aim them. Getting good/fast at loading, shouldering, pointing and getting a shot-off will not be an easier task without extra weight and clunky add-ons. I've never been in the situation of having to defend myself with a firearm (and hope I never do) but do have strong sense of how fast I can load/shoot, and how likely I am to hit my targets. Over 3 decades of shooting clays, and dozens of days hunting with them certainly plays into that. I might add, when you buy a shotgun and shoot it allot, you not only get better...but the gun itself changes personalities a bit. Runs smoother, feels lighter (=muscle memory?) you notice recoil less, etc. In other words, an untouched....uncessarily-heavy/bulky shotgun you don't have any experience shooting will not be a huge asset if the chips are down. At least, I can't imagine it would be.
Shotguns are a blast, for everything they are...and for what they're not. They do a great job of rearranging close range targets, so at least buy a case of target loads..round-up some reactive targets, and get to know that thing. I often can't go a week without shooting one shotgun or another. I love almost everything about them. Current favorite fun gun is a Mossberg 500 Persuader, 7+1, parkerized, bead sight, and nothing else. Contemplating a decent light for it...but I've been contemplating a light on a shotgun for over 10 years and still haven't put one on. lol
As for add-ons/what I'd do with that gun? (not that anyone cares) Upgrade the extractor, polish the chamber, lose the shell carrier and optic, thread the barrel for choke tubes, and shoot the heck out of it. Maybe consider shooting some clays, a discipline like skeet where you need to be good at shooting while swinging, following-through, etc. Unlikely to score well with a short barrel shotgun and you experience level, but it would be fun. Might have more success with trap. I'd suggest a standard buttstock and something like a Surefire forend if budget allows...but those come from not being a huge fan of the MagPul stuff.
I like 870s, and have had a number of them over the years. Including an 870P (all stock/no add-ons) I like them, they're reliable and super easy to work on. I hope you take these comments in the spirit they were offered.
Cheers!