Evinrude 6hp with Avon
I used a 6hp 2-stroke Evinrude on an Avon in the '80's. The dinghy had a wooden floor, three pieces, and if I remember correctly, was about 9 feet. I could plane with two adults, up to about 325 pounds. After that, sea conditions had a lot to do with performance. More recently, I had a Suzuki 4-stroke on a 10 foot Achilles and could not plane with more than 300 pounds. If you want to be sure, get a bigger engine.