I think the easiest solution (if you want to stay close to the instructable) would be to use the Funduino water sensor, but keep it high and dry, away from the actual sump. Instead, solder two wires to the electrodes on the Funduino circuit board, and dip the wire ends into the water you want to detect.
It seems that the Funduino sensor has some simple transistor amplifier, and is meant to be quite sensitive to small drops of water, or just humidity in the earth of a potted plant or such. Hence, I think you will not lead the large, multi-strip electrode array on the Funduino circuit board. You can always change the length of the free, non-insulated wire segments which dip into the water if the sensor should be too sensitive, or not sensitive enough.
Just give it a try with a glass of water (preferably from the sump, for realistic conductivity) and a pair of test wires soldered to the Funduino.