Seems like you would break the process down into bite sized decisions / questions for advice.
Have a budget in mind; $20,000 seems like a good place to start;
Bus size / length
type, for example flat nose, or conventional, if smaller, van cutaway or mid size school bus chassis;
would you want any wheelchair or handicap doors
how / where to aquire
Licensing, Titling, Registration, and Insurance concerns
Plan and design a layout and construction plan for conversion;
Where will you build / conver it?
How in depth will the build be?
Major choices for the level of complexity but also how nice the end product would be:
Spray foam insulation after gutting to exterior skin?
Major exterior Fabrication like roof racks for solar / deck / storage; boxed for storage under the bus;
And so many more;