Most advice I've seen on feeding is it is safe and recommended to use slow release every so many months or when repotting (whatever the manufacturer recommends) and/or a weakened water soluble fertilizer used every week or every watering when the plant is actively growing.

PH is another matter. I have never seen any strong recommendations on that. Neutral would be the safest bet but a lot of species begonias might grow in slightly acid soil while others are growing on limestone formations. So I suppose it depends on what you are growing. Most begonias seem very adaptable.

I've never seen any begonia article concerned about EC levels (there may be some article out there but I've never seen one). Flushing the pot every once in a while or repotting should remedy any salt build up.

Humidity seems to be one key factor for some exotic begonias. Drainage is another key factor (too wet leads to rot).