Any medium size tractor or larger can be used. The physical size of the tractor and whether it has a front end loader is more important for overall balance than actual horsepower rating. 90hp to 120hp tractors are commonly used.
The hydraulic system needs to deliver the following minimum requirements: Series I: 25-35 Litres minimum per min, and a system pressure of at least 1800 psi. Series II Kinghitter and Series III Kinghitter: 35 Litres per minute minimum, plus a hydraulic system pressure of not less than 1800 psi.
Do not fit a post driver to a tractor that is too small so it could be unstable.