I'd work on the 10% rule at that HP but a 10hp would do the job. Smaller too if needed but it would struggle against the conditions.
I'd without question stick to 2 stroke though as it's lighter, easy to store and less cost sat there doing nothing.
Another couple of things to consider. First try to stick to same makes so if needed you can switch fuel tanks (unless it's the built in tank then forget it). That way you won't need to carry an external tank, just some 2 stroke oil to throw into it when needed. Makes a big difference on smaller Ribs.
Secondly, check what weight your aux bracket can carry. Most won't hold a larger 4 stroke! Expensive mistake.
Ive a nice 2005 Merc 15hp 2 stroke available if it helps
34kgs - believe it or not it's been used as an aux on a 6.8m Rib with a Merc 150 Opti on it. Priced £995 and on eBay now. There's a video of it running too
Good luck whatever you end up with. Hope it works out for you.
Hope that helps
Peter @ Boatsandoutboards4sale ~ firstname.lastname@example.org
~ 07930 421007