Personnally, I don't think I would go for it. It looks like you have to replace everything, and, like you said, the more you go on , the more rust you will find. Let's be realistic, it will cost a fortune just on sheet-metal repair. Mine was in much better condition and it cost me $5000 just to buy what I needed, (quaters, rear floor pans, trunck floor & extentions + a couple more parts, but the front of the car, (inner fenders, cowl dash, front floor, rockers and frame rails, (front & back), were all in excellant shape, and I mean, it was just to purchase the sheet-metal, at that price, it wasn't installed. I think that, on a case like yours, I would seriously consider a new complete body. It will be pricey, but on the long run, it will be cheaper than repair yours, I am shure. Take a look at that:
http://www.dynacornbodies.com/body-shells/challenger.phpRobert