My response is not so much to the OP, because without a lot more information and detailed pics of all the rust, none of us can give them a really helpful response.
From the one picture, it looks like it's not too bad, but it could be terminal.
Newbies, before you buy a bus, take a hard look at it. You don't accidentally get saddled with a rust bucket. Most school busses are a chassis bolted onto an open frame.
Put on a pair of coveralls, put fresh batteries in your flash light, grab your chipping hammer and crawl all over that rig. Whack anything that looks suspicious with the chipping hammer and see what falls off.
Do a deep dive through the threads here and see how much work many have done on rust remediation. I am old and tired so my attitude is that any significant rust means, "run away!" Others would say, "if the price is right, there is an acceptable amount of rust that is worth doing the work to get rid of it.
Growing up in the rust belt, SW NY and PA, and being a licensed PA auto safety inspection mechanic, the common term for rust in a car or truck was, "body cancer." You might go into remission with enough treatment, but the likelihood is that it will return.
There are a lot of challenges that, when they pile up, often lead to enthusiasts burning out before they get to the really fun part, living in your creation.
If you have been lurking, waiting for the opportunity to, "pull the trigger," on your first bus project, take a lesson from the contributors here who have shared their challenges and solutions to common problems that often come with a used school bus, make a list of problems you want to avoid and be patient in the purchase processes and wait for the right deal.
Things that may say, "Walk Away:"
- Severe Rust in the frame or Chassis
- Severe engine or transmission leaks
- Corrosion in the wiring harness
- Bad engine sounds
- Slipping or not shifting transmission
- Overheating or history of overheating. (I have seen busses for sale that the seller says they just replaced the water pump, thermostat and flushed the radiator. Later revealing they were trying to solve an overheating problem. You have no idea how badly they overheated and how much damage was done.)
- Whining or clunking from the differential.
This is not a comprehensive list and I am sure other folks will add on to it. My point is this: Building a skoolie is an adventure. Adventures are a lot more fun when you start with a sound foundation. You are unlikely to find a completely challenge free bus. They are big machines that have been worked hard for fifteen or twenty years, but pick your battles.
One thing that sinks a lot of beginners is the discovery that this is not an inexpensive sport. The more you can learn about maintaining and repairing your bus, the better off you will be.
A tiny bit off topic: Before you start deleting wires or hoses, do a deep dive into the end results in the many threads that start with, "Help, I may have screwed myself!"
