Also keep in mind the accuracy loss you will suffer with a smooth bore. I had a smooth bore slug barrel (different gun than you) and the foster (pumpkin ball) slugs are a lot cheaper (about 1/2 the cost) , but accuracy limited me to 50 to 75 yards tops. I missed a deer of a lifetime because of that stupid barrel/ammo. I went out that weekend and bought a fully rifled barrel and scope and new ammo. Yes the slugs are more expensive, but I nearly doubled my effective range. As few rounds I shoot every year (beyond getting the scope set) - it was all well worth it.
I would suggest looking on classifieds and the like as many of those guns came with a smooth bore slug barrel when purchased and many folks upgraded and now that barrel just collects dust. I would not buy a new one.....didn;t go into detail, but Bass Pro lists new barrels for an 870 at $220 as a starting price.
I bought my barrel and scope combo at my "local" bass-pro. Fully rifled is the way to go - even if the ammo costs more. Just my experience.
Not sure about your regulations, but I often used my muzzleloader instead as ammo was cheaper with better accuracy and range over my old smooth bore slug gun.