8192 watts with 128 NM of torque. AKA, at least 128 NM torque with at least 64 rad/second. As for my advice on how to power the grinder, I'd do either one of these two options.
#1: Gasoline engine. You only need a DC electric engine to power a fermenter, which can make ethanol crystals out of grass, dirt, and leaves. The gasoline engine can power the grinder by itself, since it puts out 128 NM of torque at 512 rad/sec.
#2: Steam Engine + CVT with 3 belts. The steam engine puts out 32 NM at 512 rad/sec, so you'll need to increase the torque four fold with the CVT, ending up with 128 NM of torque at 128 rad/sec. You'll need a way to pump water into the steam engine, as well as a permanent fire source underneath of the steam engine, but while slower the advantage to this method is that Steam Engine power is essentially free.
As for a bit of a shortcut on all things rotarycraft, the big thing you'll want to have is an Angular Transducer. This will not only tell you the power that an object is receiving, but also the specific amount and kind of power an object needs to receive.