Why start with a bus if your going to ditch the shell?
A box truck, or a 53 foot reefer trailer may be better.
I'm all for the full tear down and rebuild. You would also need to remove the floor and start over from the frame up.
Cost, That much new metal is going to cost a bit.
However, here in Alberta, I'm allowed to build a RV up to 10 feet wide. With 4 foot slides built into each side, your main living space would be 17 feet wide with both slides out.
If you have the funds, I would love to see the build. But metal is not cheap.
I live in the same geographic conditions as you. This is a link to my build.