Many D.J.s boast about the number of songs they have in their library. While variety is great, the fact is that they will only be able to play 40-50 songs in a 3 hours of dancing. Having the right 40-50 songs is a lot more important than having 20,000 songs your crowd doesn't want to hear. After you tell the D.J. what type of party you are having and who the audience will be, ask what type of music he or she suggests. You should feel comfortable with most of his or her selections.
Professional equipment is quite different from home stereo equipment.
Home stereo equipment is not designed for long periods of use at moderate to high volumes. If your D.J. has none of the above brands, this is a cause for concern. I remember a banquet manager telling me about a large wedding in the Morrisville, Vermont area some years ago. More than 300 people were in attendance. During the second dance song, the D.J.'s amp malfunctioned.