Congrats on your Nonsuch - I’ve lusted after one as long as I can remember. Unless you’re a weight lifter or have brutish friends, I would not even consider an 8 - 10 hp. They are very heavy and pretty much impossible for mere mortals to mount or remove. The Pacific Seacraft Flicka is comparable weight and came with a 7 hp diesel which was very adequate. Tohatsu makes a 6 hp Sail Pro with a 25 inch shaft - that would likely be adequate for your purpose. No, I don’t sell them. I have made the mistake of buying too large of an engine and would much rather err in the other direction. Happy sailing. Eddie C
Weight is an issue. The Nonsuch was built back in the days of 2 stroke motors which are lighter than 4 stroke motors. A modern 6hp 4 stroke weighs about as much as an old 2 stroke 8 hp. That is a concern with the engine bracket, it should be strong enough to support the extra weight if an 8-10 hp motor is chosen. (The 8 and 10 HP 4 stroke outboards weigh about the same.)
Typically on outboard powered sailboats the outboard is mounted on a spring loaded bracket which reduces weight concerns. Once under sail the bracket is raised and the motor is lifted out of the water. The spring makes this easier to accomplish.
When I had a 2900 pound Tanzer 22 a 6 HP was provided enough power to motor through any chop, current or adverse wind. Based on that, I think a 6hp motor on a 5,000lb Nonsuch 22 would result in an under powered sailboat in many conditions, such as working off a lee shore in a stiff breeze. For that reason I would opt for an 8 HP motor.