Our venue has said we can use their DJ at no extra cost
Using the venue's own DJ might be the easiest option for you as you don't need to find a DJ and it will cost you less, but is this a sensible choice. If you decide to use the venue's DJ you will often find a far less experienced person with lower quality equipment. Venues typically hire their DJ's based on price. They want to get the lowest price possible so they can maximise their profits, after all, that is why they do what they do. Good Event DJ's don't sell themselves short and will do a better job for you, their client. A good DJ will get to know you and understand what you are looking for. A good DJ will treat you as a friend reducing unreliability and potentially stressful events.