Hi,
considering what you can do and what you can spoil with a lathe I think a controller circuit will be a MUST for your project.
With a controller circuit you can vary stepping, select full wave, half wave and microstepping depending on the work you have to to. (Making a conical shape using full step operation the cone won't really be a cone.)
There are controller circuits having been designed for stand alone operation, hence you don't have to care for a microcontroller. It's all built into the chip.
I do not suggest to use bipolar stepper motors (2 coils with four wire connection) as the one you are planning. There are equivalent unipolar stepper motors with the same torque and speed parameters as that one.
The disadvantage of a bipolar stepper motor is the fact that you need two full H-bridges to control motor rotating direction, meaning eight power transistors wired logically to prevent shorts between upper and lower branch of the motor supply voltage with the supply voltage being reversed on the coils for reverse rotation.
A unipolar stepper motor has five to six wires with 4 coils (2X2 coils in series with a center tap wired to the positive branch of motor power supply) and changing direction is done via the logical sequence of controlling four power transistors without the probability of shorts.
Finally the holding torque is not the decisive criteria for your project. The tool support moves via a worm drive which can only be moved by the driving axle, no matter how tough the workpiece might be.
Boncuk