Good point. Modern 4-6hp outboards are typically all around 60 pounds. (My old Evinrude is 38.)The reason I recommended a 6HP engine over a 4HP is that for most manufacturers 4,5 and 6 HP are the same engine (size and weight) but with a different carburetor and a higher degree of tuning. Obviously the 6HP will cost more but the extra power may come in handy in foul weather.
WRT tides and currents, I may be wrong, but I've always assumed hull speed doesn't care whether it's the boat or the water that's moving.