As a point of reference, I have sailed 26' water ballasted Macgregor for 15 years. Ballasted and provisioned, it displaced about 4000 lbs. I drive it with a 9.9 hp Johnson which seems to be plenty. At full throttle, the boat wobbles off course and is impossible to steer.
I've crossed all the Great lakes in it, sometimes under motor. Never a problem. Also, I have motored (or at least tried) into 6' seas. As the boat pitches over the waves, the prop leaves the water and I lose drive. I don't think there is any way to avoid this. You can't get the engine low enough to keep the prop in the water on a crest and keep the engine from being drowned in the trough. It's a rare occurence though. The only times I have experienced this is when attempting to motor into a river with wind opposing water flow. In open water with high seas, I'm under sail.
I don't know which engine is best but be sure parts are availble for whatever you buy. I have an '80s vintage dinghy motor that I can't repair for that reason.