I agree. My classic started life with an older 9.9 Johnnyrude, ran about 15mph with two men & full gear. Not fast but very economical and got the job done. Moved to a 25hp and will do 29mph with the same load.
Point is, if you don't need speed 9.9's are affordable and good on gas. A 15hp will do very well. 25hp is overkill, but fun to have.
Keep your eye out for an 18hp Tohatsu. Supposed to be best power/weight and bulletproof. Perfect for a Gheenoe Classic to keep the shallow draft and remain stable at WOT. If it were an LT25 I'd say Merc 25 2 stroke.