RE: Minimum Power Requirement
I was the pilot for a heavy lift model in the SAE Aerochallenge. We flew a 33 or 35# gross weight model with a .61 engine. So I pretty much believe your estimates. In-flight drag may be even less than you calculated. The most challenging issue was thrust to overcome the mass on the takeoff roll; good old F=Ma. Once up to speed the model was relatively easy to fly. I'm the wrong kind of engineer to give advice on your methods of analysis.
Chuck