So I took one of my BOB PCBs (normally used as a Break Out Board for Stepper Motor Drivers in PC driven CNC conversions) and made use of one of it's many alternate, err... uses. By adding two jumpers and some pin headers, it becomes an Arduino Shield with an LCD display, some switches, a cable for the stepper motor driver, and a relay driver for the water valve. For now, I'm using one of the THB6064AH drivers (which is sort of like using a nuclear reactor to fry eggs with this little NEMA 17 motor) but it will probably get replaced with some other small driver.
The Arduino is programmed to display message "faking" the steps I think the robot will take, although the only thing it's actually doing at this point is rotating the wiper. There are some issues with the wiper getting stuck, but that's the sort of thing you expect to find and work out.
Still a long todo list: Need to mount the driver and control board somewhere. Add the components to the BOB PCB for the water valve. Hook up the flow rate sensor, light source and sensor.