I have built a few circuits like this that have been operating for upwards of 20 years now. One section of a 4011 quad NAND gate can be used for the sensor amplifier and the other 3 sections can be used as inverters to drive whatever output devices you want.
A piece of PC board with interlocking "fingers" etched onto it makes a good sensor plate.
If you want a ready-made solution, converting a commercial bedwetting alarm might be a possibility...
