Stepper motor control

Need a bit of help for a new project. I am using a LOGO 6 12/24RC to auto start and stop a diesel water pump to prevent unattended flooding in a basement. I intend to use an inductive sensor to monitor rpm. My problem is this. I need to move engine stop lever to 'START' position first and then when engine has started and stabilised I then need to increase the rpm to full to pump the water. I do not want to use 2 solenoids if possible due to the more complex mechanical linkages on the throttle quadrant. I was thinking of using a 12v DC stepper motor with a screw thread assy for linear motion. This would have 2 lots of input wiring from 2 logo outputs to allow for reversed polarity for change of direction (there would only ever be one polarity sent to motor at any specific time - I do not need to oppose the currents to maintain the throttle position). The outputs would be dependent on float level switches and monitored rpm (AC voltage 0-approx 8v at full rpm). How can I get the LOGO to output a pulsed DC to the stepper motor dependent on the required rpm? Shut down movement would obviously have to close the throttle and stop when rpm = 0. Any help greatly appreciated.

Russ