A 9-window is enough smaller and lighter than a full length bus that the 5.9L/ISB will really move it along right smartly. At least I assume it has the Cummins 5.9L/ISB.
Some had the Cat 3116/3126 /C7 which sure is not anywhere close to my first choice. But, if the bus was purchased for a low enough price the extra cost of owning a Cat might make it a very good choice.
The only advice I would have for you is to ask a lot of questions. And always remember this is supposed to be fun.