Since these pto’s engage by the clutch pedal, it is not really an issue of hp to get them started, you just ease into it the same way you would when rolling away from a stop with a truck that was towing something as opposed to empty. Once it is spun up the HP matters, but it only matters to how fast you can go while cutting a certain amount of material. If you are cutting thick grass down by several inches you will likely need to roll pretty slowly with the tractor. If you are cutting only a couple of inches of thinner material you can probably go as fast as is comfortable (which isnt very fast considering that lifting one of those mowers makes the front of these tractors super light.. you may need to consider how you will add weight to the front to retain ‘steering authority’).
So dont worry too much if you cant find a 3ft rotary cutter.. those would be incredibly rare. 4ft is fine as long as the rotation directions match between tractor and implement, and it isnt overbuilt to the point of being so heavy you cant really drive with it.