The answer to the original question is "the maximum that the boat is rated for."
A buddy has an old riveted 14' with a 15HP 4-stroke (fairly new), and at most flows, has a hard time getting above Carver. I think going upstream from Lewis & Clark on the Sandy would be do-able with a similar setup, heck, maybe even with a 9.9 (which negates the need for a boater's card, which isn't hard to get, nor is it at all expensive when talking about buying a boat). I'd definitely stick with a prop, and learn the levels at which you can avoid grinding it (which I think is much easier on the Sandy, but there's only a limited stretch you can run a power boat -- from Dabny to the mouth, whereas the whole lower Clack is fair game, unfortunately).
Then, when the tribs are to low to motor up as flows drop in summer, you have a little boat to bump around on the Willy and Big C.