As mentioned the tires are same as what a class 8 truck tends to run. Read up on retreads. I don't really like retreads on steer tires but would not mind em on the rear duals, I worry a lot more about the age of the tire base than I do the tread coming off but everyone has to decide for themselves.
When they say it needs work on the steps, look at that and the other areas around the wheels very closely.
If damaged in an accident, that should be visible.
If rotten, need to see how bad and be honest about your skills.
Read up on the floor of school buses, tends to be rubber stuff, wood, and rubber stuff. Lots of pics of people having to replace old wood if you run some searches.
Basically if some water started damaging the floor of the bus you might be in for a project.
As I recall most steps were metal but if damaged they could have let water get into that area of the floor.
I drove a school bus for one school year and I worked as a mechanic for a bit, this was decades ago. Gassers tended to be preferred on the highway because they could wind out and go a bit faster than the diesels. The diesels did fine overall but tended to lack the top end a gasser had. Don't recall mileage at all, we had a gas card and just filled em up at the stations the card worked for.
It is a medium duty truck chassis and I would research the body builder of the bus, such as bluebird or whatever, because I recall us having to get some specific odds and ends from a parts supplier for the bus body but most stuff was easily found at a napa truck store or an international dealer for internationals.
I picked stepvan because it is what I stumbled on in my price range. I would not mind a schooly but I will admit I don't like the interior height of most school buses, I can't stand up straight and especially if I get into insulating things much.
Here in tn with my insurance company they just type in the vin number and the stepvan comes up commercial, it is a 14,100 lb gvw chassis so I don't know what it is that they are seeing since it could be that or the stepvan.
Anyway, got a commercial policy and I signed paperwork that says I do not use it for commercial use. I got liability only cause it is paid for and that fits my budget. Not too expensive but did take a bit of work insuring vs normal stuff.
Have sort of asked about the rv conversion for insurance change and basically with my insurance people I am in a rut with the vin number saying it is commercial.
Call you insurance people and ask around. I ask mine all sorts of stuff and they find it somewhat amusing at times, but overall that is what they are there for.