If you're going to be running the rivers a lot, do yourself a favour and build one of these. Also, a stainless prop can be a darn good investment.
I run rivers big and small in by 14' Zodiac and by 20' freighter canoe with this 20hp on it - no problems at all.
Those commercially available prop protectors are not tough enough to do their job in my opinion. For the price of a 20 dollar garden fork and a couple hours with a welder, you'll be able to rest at ease that your prop and lower unit wont be taken right out if you do hit a gravel bar when you're up on step.
As for whether 15hp is underpowered, I know that I wouldn't want any less than my 20 on a 14'. Being underpowered is never a good thing when you're fighting currents.