EDIT: Sorry, didn't mean to drag up an old post. I read the original posting date wrong.
I am not an expert, by any means, but I have dabbled quite a bit in the hot rodding world of sheet metal fabrication.
My opinion: if its me finding this, stop everything. It's a bus, not a rare antique vehicle that you'll never find anywhere else. Don't spend another dime on it. If the mechanicals are all solid I'd try to make some money back parting it out and find another one for your conversion.
If you're attached to it, I suppose it is possible to refurbish it, but it would take a lot of work, money and dedication just to get to a back to where you thought you were at before finding this mess.
It's a shame, and I'm sorry you had to figure this out after you already paid for it. This specific reason is exactly why I was a total NAZI about the inspections I made on prospective busses. Next time, I suggest you bring a magnet and a Small screw driver. A magnet that loses its ability to stick can show you where bondo was used. The screw driver will show you where the metal is thin in floor boards and what not.