I am shooting a festival event tomorrow, and I am wondering if my camera gear will be adequate enough for the event.

Right now I only have an 18-135 that is suitable (the other lens i have is my 150-600), I did shoot another event with this lens and all was fine because I was able to get close to my subjects. It should be the same at this event too, but the event is from 5-10, and I am concerned I wont have a wide enough aperture to compensate for low light. I am wondering if I should either 1) get a speedlight, 2) pick up a cheap nifty fifty 1.8 (which I don't think would be an appropriate focal length). I am going to this event to photograph a DJ, and based on this event I will be getting paid to shoot her events in the future so I do not want to drop money for something under time constraints, as I'd like to purchase a very nice lens in the future.