I would say nothing more
If money were no object for me, I'd get a Tohatsu Sail-Pro. It's a 6HP, long shaft engine with the high-thrust prop and generator already included.
On my C22, I have a long shaft Tohatsu 5 with the generator and high-thrust prop added as aftermarket options (some by me, some by the previous owner) so mine is nearly
a Sail-Pro, but it probably was more expensive to do it like that in the long run, and I'm missing that extra horse.
The 4, 5, and 6HP models are all the exact same block. The only difference is that the carb on the higher HP ones allows the engine to develop more RPMs, but they all weigh the same. Modern Mercuries and Nissans in this size are also rebranded Tohatsus.
The bigger considerations than HP in my mind are the shaft length and prop. Most outboards in this size are meant to get small, light boats (like dinghies and little fishing boats and things) up on plane, so they have high pitched props for high top speed and short shafts since rowboat transoms aren't that far from the water. Your C22 will never get on plane, and will never go any faster than the hull speed, so you're better off with a lower pitch, high surface area prop to get your boat moving. The long shaft is to keep your prop in the water when riding over waves.
I love my elephant ear prop.
Anything over 6HP and you're just wasting gas and making your transom sit lower in the water, in my opinion.