I can solve this task. I am an electrical engineer with over 20 years experience. I have recently worked with Raspberry Pi to control motors and read input from a variety of sensors. I am well versed in electromechanical hardware and software, API interfacing, etc. I've built many hobby robot projects and home automation projects.
This project is well within my abilities. I would need you to either acquire parts and send to me or be re-imbursed for acquiring the parts myself. I can source all of the parts easily based on your description (Raspberry Pi, stepper motor controller, stepper motors and some sort of actuator for the irrigation. There are several options for the irrigator. I would need a bit more information from you to recommend, like rate of flow, pressure, environment (temperature, humidity, etc). similarly, I would need some specs on the motors, what kind of load you need to move, do you have a gear box (you will definitely need one with stepper motors to move even small loads). I can advise you on all of this.
Another thing that I offer is documentation. Others might say that they can get you this, but I will deliver. Assembly and setup instructions with diagrams, schematics, technical docs. And I will be available to support you in future to add features or fix bugs.
If I'm selected, I would like to do a video conference to collect more specific requirements, get to know your expectations, etc.