Matalla disk diffusers have a built in check valve. The disk covers the hole at loss of air pressure. I think some other brands do too.
You will not be sorry having electric at the pond. You might need to charge your cell phone while fishing.
Even though I mostly use battery powered tools for things like dock construction, still is handy to have electric near for the chargers, etc.
As far as hose size, adequate is for sure better than too small. But bigger than needed just wastes money but will not hurt the installation. Flow (cfm) and distance is what determines hose size needed. Someone on here will have access to a chart or rule of thumb.
Oh, and welcome to the forum.
