Power for a 3.4 Mac Boat

Hi
I have recently acquired a 3.4m Mac boat, a rotationally moulded polyethlylene dinghy. It is 3m on the waterline with 5 degree V at the transom and 110cm chine beam. It seems to have a huge volume for its size because there is quite a lot of flare in the topsides above the chine. The wt of the hull is about 60kg and all up with crew, gear, motor and fuel (and hopefully a load of fish) it will be 200-300 kg (depending on 1 or 2 crew). The craft is rated to 10hp, but since I am using it off the beach, a smaller lighter motor would be easier to lift out of the trunk of my car and hump down to the water. I only want to do trips of 2-3 nm in it. How much HP do I need?
Advice appreciated.
Thanx
DEF