After looking at the link you posted that is the same model as the one I have with the Ghost Ring sights. The ghost rings on it is very good. A good set of ghost ring sights alone would cost you over $100 and another $100 to have them installed. A Remington 870 that comes with a set of ghost rings would cost you at least three times what that gun is selling for. That's a deal on that shotgun, I would go for it.
It will make an excellent HD shotgun. Mine will cycle any 12Ga 2.75" and 3" ammo. I get good buckshot patterns with Winchester Super X 00 Buck 9 Plellet buffered shot. All shotguns are different so you will need to pattern test your particular shotgun. Try out different brands of buckshot but you will probably get your best tight patterns from Winchester, Federal or Remington buckshot. The best buckshot I have used in all of my shotgun is the Federal Tactical Flite-Control, but it is primarily a Law Enforcement round. Regular stores don't sale it but some police supply stores do. You may also be able to find it over the Net. The model # is LE133-00 (9 Pellet) and LE132-00 (8 Pellet). The LE132-00 8 Pellet is a reduced recoil round and it is very good for HD if you have a problem with recoil.
Federal, Remington and Winchester all make a regular low recoil buckshot round.
Any quality buckshot will work as long as it patterns well at 15 - 20 yards.
GC