I was also thinking of the 85, 1.8 and, depending on the size of concert venue, or how close you can get, a 50mm or 35mm if you can stand just by the stage. If you have the possibility to rent, perhaps also rent a second body so you don't have to change lenses. Otherwise, the 50mm 1.8 is very affordable. And the 85mm 1.8 is a lens you will quickly fall in love with (I did).

Lars

P.s. Will you post some pictures here later?