Hi Lars
sounds like you have a bit of appreciation to go.
Don't worry about the power drains except for, around the bar-talk.
The belts, xmon, T/R system, fan, magnetos and everything else in the engine accessory gearbox all have friction loss and power requirements. Why even including disc angles and the excess power required by that all add up to whole heaps of sleepless nights if that is what turns you on.
The horsepower is produced at the top end of the cylinders, (that's the bits just under the spark plugs) and everything else costs.
The POH conveniently sets out the H.P required and or allowed to fly the A/C with an easy indentikit method of selecting it. It's by reference to the one round BIG dial right in front of your mark one eyebal, called a manifopld pressure guage. There is another simple correction to help you use it which is on the chart right above your scone with temparature conversions etc.
Frank recommends that you select a maximum of 124 HP cos I guess he has figured out how much you need to safely overcome gravity and have the engine and other bits not get all choked up with heat and emotion.
You may even find that all rotary recips use proportionally the same cooling power, which is set to keep the damm thing cool at max continuous power.
The only thing other that you need do is make sure that at flat pitch and 104% the engine does not use excessive power.
I know what that is for me.
For your own good you should find an engineer to tell you what it is for you in your climate, and if that is ok, then go.
The rest is Bollocks.
Now I know where english beauracracy came from. one of your forebares in a long boat.