How much time? What lighting and what cost? Are monograms included? Microphones?
Some DJ companies have many DJs. Which one will be there for your event? Do you get to choose? If so, is there an additional cost?
Does your perspective DJ have references and will they make those available to you? Remember, anyone that is selling something will tell you wonderful things about themselves. See if they can back it up with meaningful feedback from past clients.
Will they be there early? Will they be set up before guests arrive? What is the fee for going over the time specified? Does setting up count against the time? Will there be music for dinner?
How far does your DJ travel? What is the travel charge? Are accommodations required?