After determining the fan clutch was bad, fan would not increase with revs & you could stop the fan with a magazine, I've removed the fan/fan clutch assembly & realized a blade is missing from the fan(I suspect a years ago incident). The question is should I replace the fan or is one blade not that important. (1999 e300)

replace the fan. it's only like 40 bucks or so. a missing blade will make it unbalanced, putting lots of stress on the water pump. a leaking water pump will result.