There are tons of things to consider when looking for any musician in a band. Singers since they front the band need stage presence, be able to stay in tune and timing among other things. For me I think vocal range is most important. To be able to pull off that low raspy blues ballad and then cover a screemin rock tune are the key to longevity in the music biz. Only examples I think of with these type of voices at the moment would be Robert Plant or Mick Jagger.