I would not use peg board for the walls. Not sure what they are like now, but in my darkroom, all but one wall was just framework with insulation over it. I just nailed in a bunch of garbage bags over the framework and the plan is to put ply-wood over that. If you want to use peg board, i'd put it over plywood or sheetrock. just make sure you make a frame around the edges, using 1x1 or something, so that it can stick out from the wall a bit. Personally though, i'd rather have some nice shelves on the walls than peg board. I don't find that a lot of darkroom stuff really fits on peg board hooks real well.
-Austin