I'm more concerned about the performance on an inflatable. that raft will move like wet carpet, they take a lot to get them going. I've used a 4hp outboard and top end I would say was about 3-4 knots max. and that's being optimistic.
Personally I would go with a 45 lb thrust minimum, if you want range double up on batteries before going out. don't expect speed, range will be what you're after. be sure to bring an oar or two just in case
