The OP says "basically river cruising." What are the speed limits on your local river? On my local river, the Trent, even a 6 hp would push that boat considerably faster than the speed limit.
That's not to say that speed limits are always observed or enforced, of course...
There are 3 approaches depending on your personal preference and finances:
1) Get the biggest engine you can afford that is within the manufacturer's limits for the boat. It will be expensive to buy and insurer, and heavy, and will only be used to its full capability for part of the time
2) Get something middling around 10-15 hp which is reasonably cheap, fairly easy to carry, and fast enough most of the time.
3) Get an egg whisk (3 hp or so) that is cheap to buy, insure, and maintain, easy to carry and store, good for pootling about, but may be a bit noisy.
I've had as much fun with option 3 as I have with option 2 (I have one of each). I've never had the funds or the inclination for option 1.
2 strokes are generally lighter, easier to maintain, but noisier.
The dirty/polluting aspect of 2 strokes is a bit of a misconception unless you are using it in a very sensitive ecosystem. For the very few hours most outboards are run each year, the extra environmental cost of manufacturing the 4 stroke outweighs the environmental cost of running the 2 stroke.