Here is what I had to do to repair the rusted-out floor on my bus. Your bus is not nearly
as bad, but you will still have to do a fair amount of rust remediation, which will include cutting away the hopelessly rusted parts of the floor and welding on patches. This will be a lot more work than the
nothing you would have to do if you were to start with a completely un-rusted bus.
Converting a skoolie is a heck of a lot of work as it is. You shouldn't add significantly to that work load unless you really want to learn metal fabrication skills - something that a rusty bus is perfect for.