For practical purposes, I don't think you would notice the difference between a 38HP and a 43HP engine.
Depends on the power curve, how well it is matched to the prop and gearbox, etc.
By "gates", perhaps you mean "paddles"?
Just learn to use the paddles. There are locks that have a strong "draw", and others where you can just whack 'em wide open.
My boat is a 55 footer with a 27HP engine. This is perfectly adequate on the canals, and perhaps a little underpowered on the river.
IMHO 38 HP will certainly suffice.