From what I've been told about the 9L engine, true, they have become a rare breed, but as long as they are not run hot, not pushed too hard and oil is changed regularly, they can run a long time. True, they have a reputation as "boat anchors", but from all the research I've done, most of these stories involve vehicles that have been pushed way beyond what the engine was designed for. Even when being sold new, many dealers were instructed not to sell these engines for medium or heavy use. International knew the limits of the engine. Used in a bus (as long as it's not in a hilly area) they will serve well. Put one in a tandem axle dump truck and run it up some hills, and it's not going to pull well (and probably not last long, either). My dealer friend told me of a fellow that had a pair of 'em, ran 'em for many years with no trouble. He was known to take care of his vehicles and not push 'em to their limits. He finally retired and had some younger fellow start driving in his place. One of the trucks soon suffered some sort of catastrophic failure (I forget what), showing many hundreds of thousands of miles. The other truck was pressed into service and soon began blowing head gaskets. It was determined this driver had figured out how to turn up the max RPM's on the engine and was pushing it too hard.
My mechanic friend (from who I bought the bus) tells me that in all his years, the only work he has ever done on a 9L is replacing a water pump.