I think any good vent fan would work well. You should think about filtering the air coming in, though to remove as many of those icky particles that make developing such a wonderful thing. My hubby built a light safe passive air intake for my darkroom & I se an air ionizer, but mine's in the basement (no worries about hornets or other wonders of nature visiting through the slot! haha

We are on a septic system too, and I mix my used developer, stop bath & water together & dump it. I am amassing my used fixer while I try to find some way to get rid of it that will allow me to sleep at night! I cannot believe how many pro photogs locally I have had tell me they just dump it....I am aghast! :o