Being a 2001, that 5.9 surely must be electronic controlled, in other words, the ISB engine. I like 'em fine, but plenty of people here prefer the older mechanical injected kind. That said, here's one point where the electronic can actually help you out: the ECM tracks engine operating hours. IIRC it also tracks mileage. If you can find somebody with Cummins INSITE software and an interface cable (not OBD2) they can pull that information from the ECM and advise you whether the mileage on the odometer is plausible or not. I think there was a flag in my ECM indicating whether it was original or replacement as well, but it's been a year and I don't remember all that well anymore.
In the case of my bus, I learned (some time after buying it) that its mileage was respectable at 287,000, but it had 21,000 hours on the engine! Ouch! Must have spent a LOT of time idling.