I'm going to take a different view on it and suggest that you not buy it at all. I certainly wouldn't pay $850 for a 24 year old Citori 20 gauge with fixed chokes, particularly modified and full. :!: What the heck are you going to use it for? Having fixed chokes of modified and full certainly limits your practical uses for the gun.
For just a little bit more money you could buy a brand new Browning, Beretta, Ruger, or Weatherby/SKB of any gauge you want and with screw-in choke tubes for a great variety of uses. Or, I'm sure there are newer used guns available at about that same $850 price which have the screw-in choke tubes.
I don't know what your experience is with shotguns, but if you are fairly new at it, don't be in any hurry to buy something. There are plenty of guns of all types available for sale. Don't think that you have to buy the first one offered to you.