Waste Water Lift Station - Wet well exhaust system

First, I would say that plastic would generally be better than sheet metal for that type of application. Second, is there a particular reason that you feel that active ventilation (exhaust fan) is necessary to exhaust from the wet well. In all applications I've seen, a vent is provided (usually a DIP or steel pipe with a gooseneck and insect screen) and that's it. Unless there's a special condition, an active ventilation system is overkill and (as you guessed) will be a maintenance nightmare.