ditto what BigBear said
either a moss 590, 500 defenders are ok too
or rem 870, i would prolly reccomend the 870.
getting small parts for the 870 would be easier, ive learned that mossberg kind of does things there own way.
like the bead sights are off thread, and the mag tube is a slighty smaller, we tried to fit a 12ga end cap for a spring in a mossy and it wouldnt go into the 12ga tube -- weird.
only bad thing i can say about the rem870 is the ejector can break, but only after constant long term of storing the gun with the action open and after many years or hard use. replacement is expensive since its rivited in, ive only heard of this on duty guns that are stored open, and never on a civie gun that gets proper care, so dont sweat it.
my friend took a shotgun instructors course with a bunch of cops and there were a lot of mossys out there, but most had the vang comp system.
keep a watch on the turners site, rem 870 special defense model comes on sale about once a month for about $300. its ready to go with bead sight and tube extension.
forget the top folder - its only useful in the movies, and if storing your gun in a small space is required.
my brother has a side folder and its neat but added a lot of weight to the gun, making it not so fun to carry.
folders and P grips are legal in kali on pump shotguns, and fun to play with but arent really used for serious play.
look at what the cops are using and go with that, since they are the ones who use the thing on a daily basis and have to pass shooting classes.
go with the tube extension, the rem and mossys already got it.
you have to use a short 4 rnd side saddle for the rem870 with long handguard, or change out the handguard for a short one and then use a 6 rnd saddle, changing it can be a bitch.
then a buttstock shell carrier and a good sling.
get profficent with that, before adding on $100 ghost rings and $300 vang comps.
you can buy the pistol grip only and play around with it, but dont bother with the folders.