Welcome to the forum.
Having owned a "normal" camper, and now owning a bus that I'm converting, I can say that unless you buy a really high end camper (ie, a wanderlodge bus or similar), you'll find the camper falls apart rather quickly. The bus on the other hand is built like a tank.
When looking at busses, don't trust the odometer. You never know (unless you have full service records from the school system) whether that engine got replaced at some point. It is quite common for an engine to have been replaced.
The Thomas flat nose busses are very good busses. You didn't say whether you were looking at a front engine or rear engine bus. A rear engine bus will ride better, and the engine noise and heat are far from the driver during operation. But, a front engine bus is still a good bus.
I'm not familiar with the 5.9 International engine, so I can't say much about it. The MT643 transmission is a nice transmission to have. The 86 passenger capacity would lead me to believe your bus is close to 40' long.
Thomas seems to make 3 different roof heights. If you look at the bus from the side, you'll be able to tell which type it is. The one with the lowest interior height has the roof lower behind the driver's area. The next height up would be straight back from the driver's area. The one with the best headroom is the one that rises right behind the driver.
There have been many nice conversion of school busses. Take a look around in the gallery, and look at the various conversions posted in the forum. Conversions here run the scale from "mobile tent" to "mobile mansion" and everything in between.
I hope this helps,