Welcome to the site Ian.
Many of the following ideas you might already have in mind, but it bears repeating for anyone else reading this thread that might be new to the hobby.
I would suggest finding a candidate bus with little to no rust/rot underneath.
Then focus on the functionality of its running gear. You don't want to buy a bus only to find it needs extensive, unplanned, mechanical repair work.
While some might argue that finding such a candidate is impossible, let me tell you from experience, it is not! You simply must be prepared to walk away from one that doesn't fit the bill, and keep looking.
It took me the better part of 6 months, looking at dozens of potentials, to settle on what I have now. Six months of searching isn't bad, in the big scheme of things.
A bus that has seen its use anywhere in the nations "rust belt" should be eliminated from your list. Unless all of the rusted structural members have already been repaired or replaced. Then it would take someone with a basic knowledge of vehicle construction to determine if the repair(s) were done correctly. Duct tape over rusted out wheel wells doesn't count!
You might look at a bus from the rust belt that has a good looking outside body. However when you dive in and discover the floor, wheel wells, back door/wall, etc. are soft enough to poke a finger through, your project has just come off the rails.
Hope I haven't bored you with minutia on rust, but it can be a real bummer/project killer when much $$ must be spent to get it safely back on the road, and the actual camper conversion process hasn't even started.
Good luck and remember its always "Buyer Beware"...