Even though it is a metal bus you
should still strongly consider The clearances to combustibles like: Painted metal surfaces, and cupboards etc. Especially even glass windows. When it is cold outside and your hot barrel is close to your window it could shatter. If you could find a place in the bus that the barrel would sit 18" from anything. Then that is where I would put it. Also put some type of commercial fire barrier on the roof, above the barrel. essentially you are putting a barrel inside a barrel, the second barrel being the bus! And temperatures above the barrel will be the highest surface heat from the rocket.
If you can you should insulate and seal the bus, rocket stoves suck air out of the room when heating, and blow air in when cold. If the inside of the bus gets colder than the outside air which can happen during an initial burn on a cold
rocket stove. You could end up with smoke back and in a smaller room like a bus that can be annoying, and also dangerous. I would consider having an external air intake. Also any cracks in the door seals or window seals will cause cold air to be sucked in by the rocket stove, and certain areas may feel draftier than others.
I found this video to be helpful in understanding why an external air intake is necessary in a small drafty place. I am not saying your bus is small and drafty, My imagination tells me buses are small and drafty from my experiences of sitting on a bus in my childhood. I am not sure what you mean by commercial bus? Is a Commercial bus like public transportation or a school bus? Those are commercial buses. Or is a commercial bus like a greyhound bus? All of these buses are used for commercial enterprise.
Finally don't use metal use firebrick or castable refractory for your heater core.
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