The problem with buying a partial is that there is lot of critical steps that are the first you take. To accomplish this as a "recovery" you will have to rip out all that has been done and then fix the problem and then rebuild. It is most often easier to start at the beginning and that certainly will take less time and effort. So much for philo 101.
There is a "return air duct" built into the bus and it runs down the center and under the floor. Most buses have the floor removed as a part of the conversion so that this "horrifically fouled and trash infested" area can be "shoveled out" then vaced and finally pressure washed. And then the new floor is installed. This can be done by carefully cutting a section out of the center isle but I don't think many have done it that way. If you don't do this you will find that in humidity the interior will reek of mildew and worse. Think Gym Locker.
The walls have square tubing in them as frame. The water that collects on the windows interior seems to be ducted down the inner surface of the tubes and they rust away where they are welded to the floor. That is inside the wall, mind you. I have only seen pics of one bus that had almost zero rust and even that one had a couple braces welded in. Usually there is much of the framing that needs to be cut out and replaced. This can only be done with the interior wall covering removed.
INSULATION Notice I raised my voice? That was no accident. The absolutely very best is the minimum acceptable. That means closed cell sprayed in foam. Polyiso foam board backed with alu is a close second but doesn't have the sound deadening performance of sprayed. The floor gets an inch of foam board under the plywood for an added 7. Without this you can only use the bus in temperate climes. The original bus had a Godzilla sized AC unit that would cool in Phoenix but you don't have that system and you couldn't afford to run it full time if you did.
AC There is no way that a single ac unit will cool that bus. You need two 15K BTU min and you should have three so you have a "hot spare". Also a third will draw down the temp when you first get home and you can run two at the far end of the bus to make it quieter at the front or vice verse.
That bus looks to be powered by a 8V71 N engine. That will mean that almost any hill will drop your speed to 35. Good engine and you will get decent mpg but a powerhouse it ain't in that bus.
Tires MUST be newer than 8 years old. Over that and they are truly a hazard. Regardless of the depth of the tread.
Now this might surprise you but I suggest that you buy the bus. Looks clean and if the state owns it it gets really really good care. Especially in Maine. Correct me if I am wrong but they don't use salt up there. Get a qualified mech to evaluate the engine and trans. When you figure your bid just adjust it for stripping it out and fixing stuff and then you will have the value of the shell. Lets see now....that would be $12K minus 14K is.....oh dear, he will pat you $2K to drive it away.
HTH and welcome to the madness. Hope you get your bus soon....whatever it is