I think a capacitive sense would work best. Resistance is relative to salt concentration, and would require re-calibration every time. I could easily rig up a test with an arduino. you just need to tape a conductive strip to the outside of the container vertically, and submerge a corrosion resistant lead into the water. Gold coated PCB trace would be ideal, and cheap/easily salvageable. You can measure the capacitance which should increase linearly as the water level rises. You can measure it with an arduino using 3 pins, and two resisters, or using a 555 timer, to generate a digital modulated output which could be measured by your drip input on the peachy.