voltage is not important is starting, its amperage
Not really true. You can not increase the current that the motor receives until you increase the voltage to the motor. What is true is that different batteries have different internal resistances which drop the open circuit voltage of the source when under load. The speed of the motor is proportional to the voltage applied, but the power of the starter is proportional to the voltage applied to it squared.
So you want more voltage and a low internal resistance in the battery.