Welcome Jeangeorg, to the SBO forum.
You are now a member of a great family of sailors who offer each other the best advice and support.
First, listen to CrazyDave, he is amazingly knowledgeable about your boat and one of the most helpful members on SBO.
Second, use your own judgement to sift through all the advice that comes your way. No one but you knows your complete and unique circumstances.
Third, I make up life as I go along, so take whatever I say with a large portion of salt. I rarely hesitate to offer my opinion and usually I take it as a given that everyone should understand it is an opinion.
So, here it is:
Oversizing the outboard on a sailboat can have terrible effects on the boat. 9.9hp seems appropriate for a 25' sailboat. Even that is more than enough power. Sailboats are not designed for powerful motors, they are designed to sail, which means keeping weight down where possible. This often means the transoms lack reinforcement. I have seen where motor brackets have gone right through the transom of a small sailboat.
I couldn't find the picture I remember seeing of a bracket punched right through the stern, but this is from an article at
http://www.mariner1922.com/transom/ . Orion is a 1970 O'Day Mariner 19 who's transom began to crack under the weight and force on the motor mount. As a Mariner owner, myself, I participate in the Mariner Class Association Forum. Nobody uses more than a 6hp motor and most are in the 2 - 4 hp range. 2hp is usually enough to get these 1400 lb boats to hull speed against a stiff breeze in a small chop. Some owners even use a 55lb thrust electric trolling motor and claim good headway. That equates to just about 1+hp, I think (translation isn't exactly direct between lbs of thrust and hp. They measure different things).
For your 25, I would consider Roland's suggestion. You could replace the 20hp with a 6-10 hp motor and effectively get the boat for less money or a newer motor.
BTW, I see you are from Quebec, We are practically neighbors. I live an hour away from your Southern border.
Good luck and I look forward to reading more about your sailing adventures.
-Will (Dragonfly)