I recommend 316L. I installed a stainless steel drain fitting in a fiberglass sink and after one use with stop bath and fixer down the drain it rusted. I'm sure it was a lower grade of SS.
For contact with darkroom chemicals 316L is the right choice. If you are just using the sink for a water bath and won't dispose of chemicals through its drain, then 304 would be fine.
FYI, I worked for a company that made electronic products used in toll collection. The equipment was installed adjacent to highway toll lanes at the toll booths and subject to outdoor weather and the salt used to clear ice. We made our stainless steel cabinets out of 316L to avoid corrosion. The cabinets were not cheap, nor was the equipment we installed in them. We had a lower cost alternative which was powder coated aluminum. It was fine as long as the coating remained intact so corrosion could not start.