Actually there are strength tests they perform on metal after a fire to see if it is still ok. Now i have fixed plenty of fire damaged vehicles before, they have mostly been engine fires. The first one i did was a 1996 GTi years back and i was worried about the same thing, it was burnt from the A-Pillars forward and i was worried about the structural strength. I took it to airstrip to have it Blasted firewall forward, and i was assured that it would be fine. When i picked the car up from blasting it looked like brand new, i could not believe it. That car is now back on the road.
I currently have a 2003 BMW 325ci with a front end engine fire that will be fixed in the same manor, i wish i had photos of the previous fire cars but i dont. I will take photos of the progress on the BMW.
I think the bus will not be to bad, the reason the fire progressed so far forward was due to the fact that there is so much open space being a 21 window rag, it just got alot of oxygen.
I would rather restore this bus then pay double for a rusty one. It is the second cheapest one for sale on the market (samba) of all 21/23 window buses. The only cheaper one is rusted beyond belief, is incomplete and had a tree fall on the roof. Even if you purchased that one, by the time you bought all the rust repair panels, before even starting any work on it, you would still be ahead of the game with the burnt bus.
Its a BIG project, but i think the price is fair.