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.