If you are planning to go on the big water then definatly get more HP. As someone else said, 2/3 of the boat's max HP rating. You don't want to get stuck on the lake if a storm comes up & be under powered. I had this happen to me when I was fishing in a friends boat. We all put on our life vests because it got that bad, that fast. Being under powered we ended up getting stuck in some rocks near an Island because the motor couldn't handle the wind & waves..
Also you should look for a boat with a wide beam. That will help a little with the waves.