Then you have no choice. You simply MUST begin with a brand new 870P. You will lose all bragging rights now and forever into perpetuity if you construct your tacticool shotgun edifice upon any lesser platform.
You should start with the basic model 879P, the #4899. That's the blue steel, wood furniture, no magazine extension, 18" barrel, bead sight model. That's the foundation which gives you the most leeway upon which to exercise your tacticoolness, to mark this creation with a heaping portion of your very own individual desires and tastes. This is as plain vanilla as an 870P gets, and of course that means that you simply must change EVERYTHING about it in order for it to be an acceptable statement of your individuality. You have no choice but to change everything about this gun, no one save some poor rookie flatfoot who had such a piece inflicted upon him by a severely misguided department would be caught dead with such a shotgun. Either that or some troglodyte like sm, who for some perverted reason likes simply appointed shotguns with blued finishes and wood furniture. Of course Steve has likely spilled more shot than most of us have ever fired, but so what.
OK, so you have the basic platform in hand. You need a big head safety, and since you're going to have Vang do your barrel, you'd just as well send along your trigger plate and let him put in one of his dome-head safeties while he's at it. That assures you'll never miss getting your safety switched off, perhaps maybe even occasionally when you didn't intend to do it. Or you could save trouble and just buy your base shotgun from him for a mere $1300 and be done with it. But where's the fun in that?
So, once you get your barrel and trigger plate back, you have to get the rest of your metal work done. After all every piece of steel on your creation has to have the exact same finish in the exact same shade, right? Can't have any mismatched-color parts on this creation, no sir, not acceptable. So off to Wilson to get sights put on (nothing but ghost rings will do) and have everything parkerized to match. And of course you have to have a magazine extension, so get one from them while the gun is there. Nothing else but WILSON COMBAT is good enough after all.
You still have to have replacement furniture, of course. So you add a SpecOps stock for its pistol grip, adjustable length and recoil absorption capabilities. And a SureFire LED fore-end to match, natch.
Now we start getting to the hard parts. Optical sights, yes or no? As long as you have room, why not? There's still space on top of the receiver after all, it shouldn't go to waste. And a sling. Single point or three point? Oh, the humanity, what a decision. But you've about covered all the angles at this point. And you still have a little change left over out of two grand, so there are still things you can do if you aren't through with it yet.
So, now you've had the gun in your name for 18 months, it's spent more time in shipping than it has in your safe, and you have yet to fire the first round out of it. How kewl is THAT?
lpl/nc (hope this isn't what you have in mind)