The 7.3L is a solid engine. I have no personal experience with the Chevy but it seems to be well regarded.
The most common oil leak on a 7.3L is at the turbo pedestal and it's fairly easy to repair.
"Fleet maintained" vehicles that are scheduled to be retired often don't get much in the way of scheduled service and may not have had some needed repairs done. Assume that the auction vehicles will need a bit of repairing. Frankly, I'd be willing to pay extra for a bus that had been well cared for (with service records).