"...cutting the barrel on an 870..." Hi. It have a rib? Makes cutting the barrel more tricky. Has to be done at one of the rib/barrel connections. Then the muzzle has to be filed flat and square.
If there is no rib, use a pipe cutter and file the burrs off. Still has to be flat and square. Then, if you have or have access to a drill press, drill and tap for a bead. A hand drill isn't accurate enough to get the hole absolutely top dead centre and square.
Remember that you will have no choke. Changeable chokes can be installed, but it's a job for a smithy.
"...like to install rifle type sites..." Where you are maters, but buy a rifle sighted barrel. Far less fuss and installing the front sight has to be done right. They're usually silver soldered on(regular solder won't keep the sight in place. Bluing needs to be sanded off first too.) and front rifle sights have to be absolutely top dead centre too.
A barrel doesn't have to be made by Remington. Remington barrels are more expensive and there are lots of aftermarket barrels for 870's. Bought one a couple years back for $200Cdn. A Remington made barrel ran $100 more.
Don't bother with the laser, flashlight or folding stock. The former are just silly. The latter will increase the felt recoil. No pistol grips only either. Completely useless on a shotgun. No control. A stock with a pistol grip is a different thing.
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