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Anything in the 6 to 9.9 range will work fine. Stay away from the smaller single cylinder engines, they vibrate and make so much noise you won't want to use it.
I have a 16' Starcraft w/70hp Evinrude and 6hp Johnson kicker. I can go as slow as I want but I typically troll between 1.5 to 1.8 mph all day long. At full throttle, the 6hp will max the boat at 6 mph.