So, following the notes provided here:
It looks like the 20 RPM speed shouldn't be a problem. Simply put, speed is important because too fast means it will throw plastic out of the hopper, but too slow means grinding takes forever. The slower it is, the more torque you get, however, meaning you can shred thicker plastics.
Following the formula provided on the pages above, Torque (Nm) = Power (Watts) / (RPM * 0.105), a 1500 W motor running at 20 RPM will provide 714.3 Nm of torque.
The only estimates I've found say that 20 Nm should be enough to shred bottle tops, while 60 Nm should be enough to shred "the front panel of a domestic appliance". That sounds low to me, but regardless, ~700 Nm is undoubtedly enough to shred pretty much anything we want.
Of course, at 35,000 KSh, we're blowing almost the entire budget on just the motor. We could look at smaller motors. Again, following the notes and formulas above, anything around 500 - 750 Watts (+/- 1 HP) running at any speed between 20 - 80 RPM, should provide enough torque, especially if the main use case we're looking at is shredding PLA filament out of a 3D printer (it's very soft).
The other important question is whether it's single or three phase. A single phase motor is much more versatile, since you can plug it in anywhere. A three phase motor requires an industrial socket. It can potentially be adapted, but that requires some non-trivial rewiring.