Why use a solonide for the refill/topup, a ballcock float from a toilet system works very, the later designes with the vertical floats are easy to hide in the pond.
A mechanical system is far simpler and reliable over the electricla approach.
Should you want to stick with the solonide, a method i have used is a fill sonsor from a washing machine, as these can be mounted away from the water with a small tube going into the water to sense the pressure of the hydrostatic head.
For the pump if you use a sump pump they have a built in float switch which will turn the pump off when the water level drops too low, so no need for extra electronics there.
I would stay away from probes if you want long term reliability.
The addition of a small float switch to be a fail safe for the water level would be more reliable than probes anyday. (have a look on ebay for float switches lots of them cheap)
Pete.