What would be a fair price for a 496 HO, with low hours, but possibly missing some components?

Define low hours. What parts are "possibly" missing? A $2 spark plug? Or some other grossly overprices Mercury part?

Whats missing?

If you don't mind doing the Volvo / GM thing than I would buy one from Sean. He is has some for sale in the classified section for 7200. with 0 hrs then all you need is a cam and ecu reprogram and you'll be done. He can even do the cam change and reprogram for you. To me thats a heck of a deal for a new engine. Or you could buy it as is and that leaves some money for some Raylar upgrades. All I had to do was swap my engine mounts, bellhousing and coupler. everything plugs right in except for your tilt and trim wires which you have to splice a new connector on.