Instead of plywood I would build a frame work out 2x2" then nail old pallet boards to the frame work then if you want doors you could cut out a pair of swinging doors for the opening.it a cheap way to do it and looks old.and plywood is so spendy anymore.And as far as Ideas google old west ghost town photos.
nickanap you should be able to get pallets for free almost at any warehouse type place. Usually they through away the real bad broken ones but most will recycle the still decent ones. Good luck and post some pics.
One other tip is to take a torch and burn the wood a little to give it a aged look. Be careful when you burn the lumber. If you get to much of a dark area from the torch just sand it down to what you want.