I'm just going to drop this in for you to think about while hunting ...
Rust, a perennial problem and how to view it.
It's simply a sad fact that, until our vehicle are entirely made of carbon fiber (when they will have other issues), they will rust.
The extent of the rust depends on a number of factors, and age isn't really one of them. I have seen vehicles not much over five years old that look like cheesecloth with a bad case of moths. Others can go decades and never suffer more than minor surface rust.
Where they live, and how they are cared for makes a difference. In much of the Mid-West and North East they get a relatively damp climate and roads may or may not be regularly salted. Salt water is simply a steel killer. Even buses from wet climates can survive quite well if the county or city doesn't salt the roads.
The further West you go, the drier is the native climate. Humidity can be as important as actual rain. So anywhere in a line through the Dakotas to Texas, and West of there is generally a good bet. If rust can't form, then the vehicles are clean. The possible exception is right on the West coast, or any coast, where salt air and higher humidity can cause havoc.
The flip side of this is that the school districts know their buses will last longer than the ten minutes they last in Michigan, so they have a tendency to spec them high and keep them longer. This means that if you are looking for the most desirable buses, then the further West you go, the higher the chance of finding one. Mileage may be higher and they could be older in terms of years. In terms of corrosion, many are not far from factory-fresh.
Secondly ... if possible, get one that wasn't bought and used as a route bus, carrying kids daily from home to school. Most schoold districts have "special" buses that were bought as Activity or Trip Buses. Again, these were higher specc'd and used for transporting sports teams, the band, etc around the state for fixtures and competitions. The advantage here is not just the spec, but the fact that the mileage will be lower and the driving patterns better suited to the drivetrain. In other words, even with the higher mileage there is a fair chance that the engine and transmission is in better condition.
Into the category of Activity buses you can add Bookmobiles, Bloodmobiles and Police Command Centers. They often come with other nice goodies like generators, air-conditioning and self-leveling systems.
So your choice is to search for a rust-free (ish) bus and be prepared to travel to get it (I went 500 miles but would have gone 1500, just factor in the cost and pay, or bid, accordingly), or accept a rusty bus and be prepared to remediate the rust. The good news here is that buses are simple, if large, beasts and the work is not difficult. It's almost all labor, and if you are doing the work then it's just part of your hobby.
You need to walk away from severe rust on the undercarriage, especially the frame. As long as the metal doesn't fall away in chunks, you can rescue it. You will be a reasonable welder when you are done, a useful skill to develop.
The advantage is that those buses are super-cheap. Dealers don't want 'em, nobody wants them and if you are bidding at auction you will freeze out the chancers and the scrap dealers really fast (They are anyone who bids in the first two weeks of a three-week auction, unless someone whacks in a high bid).
So for anyone buying ...
You need two bus specifications.
The first is the ideal bus. This has everything you want in a bus. It's the bus that, if it fell into your lap you would sell your first-born to acquire.
Then you need to decide, in each area of the bus, what compromises you can live with and what compromises break the deal. For example ... You want a Cummins 8.3L engine with an Allison MD3060 transmission ... but the Cummins with the MT643 would be okay ... or the DT466 with either would be doable.
Once you have those two buses in mind you can start to look. You will immediately dismiss about 70% of everything you see and that saves time. Then you can get serious about the examples that seem like they might work.
Remember that a bus in the next county is worth $1000 more than a bus 1500 miles away. A bus with fresh tires is worth at least $1000 more than a bus that needs tires before you can drive it home. Declared engine and transmission issues are, with a few exceptions, buses to walk away from. You might be able to fix that non-starter for a few hundred dollars, and it might need a new transmission ... without an inspection by someone qualified, you just can't tell.
Finally, if it is later than 2004, walk away. You do not need to be dealing with diesel engine emission controls until you are very familiar with diesel engines ... and even then school districts are selling them rather than trying to fix them. They know way more about the bus than you do, so if they are selling a bus less than 15 years old, take their advice and let someone else buy it
There is other stuff ... other people can add to this if they want, or correct my mistakes.
Thanks for the bandwidth.