A few thoughts
It sounds like the advise so far has been good. In addition, I'd give great weight the conditions you're likely to encounter. While a 4 or 5 HP is probably fine, I might move to the higher side if I had to contend with currents...as in a river. Another important thought might be concerning service. Unless you're a DIY guy, knowing that a good Mercury, or Honda, or whatever shop is conveniently close by might tilt your decision on brand. Be sure to spend a few extra bucks to get a long shaft motor; you won't regret it. Good luck