Ordinary sump pumps do poorly in a darkroom environment because the chemicals (including stop bath) tend to be corrosive and rust them out. You might end up replacing pumps too frequently. You can make a good sump out of a heavy-duty 40-gal garbage container and lid. The sump pump itself should have corrosion-resistant impeller and housing. Grainger has a nice 1/3HP model with an ABS housing around $175 with integral float which should last a decade or so at least. By contrast, the typical hardware store or home center pump will probably fail in a couple of years - not exactly a bargain at that rate.