Most DJ's own a very large collection of music. You still need to make sure they have the music you would like and that they are comfortable playing it. If the DJ has no experience of the particular type of music you want, they should speak up early and recommend another DJ who does. DJ's streaming music has become much more common in the past few years. This will allow them full access to almost anything you want but is totally dependant on the internet. If the wifi fails then you have no music. Make sure your DJ owns their music and is not renting it from some streaming service. Streaming is handy in an environment where there is great wifi for sourcing the odd track they do not have but do not rely on wifi for any reason.