For rivers and lakes, or even voyages on choppy seas, I'd be tempted to go for the portability and integrated tank. If you want to spend a lot of time on the plane, then you will need a 9.9 or above to get the best performance.
I used regularly to do long distances on rivers and sea lochs with a 3 hp. It can be done easily. For longer trips, I would take spare fuel, but, from memory, I could get an hour or so out of a full tank. You will need to carry spare fuel at least until you know how long YOUR boat will go with YOUR engine on a tank full.
If you're not going to plane, then the boat has a natural maximum speed through the water, and it will achieve this on any engine around 3-6 hp depending on load and trim. Of course, if the water is choppy or there is strong wind, a 6 is better than a 3.
Much lower than 3 and you may regret your choice. My 310 only trundles on my 1.5 hp (equivalent) Torqeedo electric.
__________________
|