What does the engine do? What does the transmission do?

[OlgaAK]

In a bus, specifically. Do they need to be matched to one another in a certain way? How do I know what I need in each? What are the pros and cons of the different types?

[ermracing]

Kind of a broad question here. What sized bus are you looking at, what will it be used for, what type of terrain will be covered?

Any engine/tranny combo in a bus (coach, school, transit, van style) will be paired up so it will do the intended job. If the original intended purpose of the bus and your intended purpose match, or are similar, it should be fine for your needs. If you take a bus spec'd out for warm, flat country (like Florida), and want to trailer a heavy load over the Rockies in Canada, then no, it will not be a good fit.