Again, this is where a reputable company will be apparent form the get-go. An experienced professional will be able to explain that most DJs work with either decks and mixer, or a laptop. That they can either plug into the PA (sound system) on-site, or supply their own. And they will be happy to liaise with your venue to see if using the on-site system is a possibility, if there is one. They will be able to guide you regarding speaker size, and lighting requirements. Equipment hire costs should be fully set out on your invoice with no surprises at the last minute.