Anything manufactured after 2004 will be burdened with EPA mandated emissions controls that break easily, and are repaired expensively.
What are your goals? What do you ultimately hope to do with your bus? With only a vague impression of what you wish to achieve, a shuttle bus may be, in general, either a dream or a nightmare.
Not knowing where it, or you, are located limits suggestions of what to look for when inspecting.
The list is more lengthy in the Rust Belt than in the SW, there are plenty of threads describing pre-purchase inspections. I'd recommend reading around the forum...
Those who say that it cannot be done should not interrupt the people doing it.