Wireless water level sensor, in sump. Installation best-practices for longivity
I suggest you look at some of the marine solutions for depth sounders.  

I don't think condensation will affect your readings as they produce weak near instantaneous echos which probably can't be detected.

Can you seal your system inside a pvc or other plastic type tube which could be inserted inside the sump?  Transducers will work well as long as there is no air gap between the transducer and what you are measuring.  So if you put the transducer in the bottom of a pvc tube, and poured epoxy around it (keeping out all the air bubbles) it should work fine.  Then you could seal the rest of the electronics inside.  You could even change the lid on the sump to be fiberglass or pvc with the sensor built in.  The plastic will allow your ISM radio to broadcast outside the well.