I've got an old22 footer with a cabin, weighs about 3500 lbs bare, I think. I've used both a 25 hp Johnson kicker and a 12 hp Tohatsu, both of which do fine, the 25 better, obviously.
I would think 9.9 would push it home in fair seas, but if there was weather to consider I'd want a little more. Keeping headway and steerage in a little sea might be tough.