protip: Google "site:skoolie.net rust removal" or whatever else you seek. Between google search results and image search results you'll have a lot of good resources.
1. Wirebrush that rust away for sure!
2. Your flooring will have layers, floor, insulation foam, radiant barrier, plywood, topcoat flooring (hardwood, laminate, rubber, vinyl, etc.) I am going to use vinyl fake hard wood floors because it is hardy, will not mold, will not rot, immune to water damage. It is about 3-4x more expensive than laminate, and will last 10,000x longer. Can you imagine how crappy it would be to remove your flooring of the home you live in and put new better flooring back in? No thanks.
3.I went with XPS insulation for flooring because I have been under the understanding that XPS will perform best when wet(from an xps vendor website, take with grain of salt). For your walls and ceiling you want 2 part (not 1 part) spray foam. Don't let the high price tag intimidate you. Study up, read lots of builds and spend the extra on great insulation. Foam will add structural support for your bus and waterproof it as well, in addition to giving you very high R value per inch. Enjoy it for life. I went with polyiso foam, it's not bad but I should've just spray foamed it.
But if you're a fool like the rest of us which didn't go spray foam here's the basic breakdown of insulations.
Foam boards.
Polyiso is supposedly 2 part spray foam sprayed into a mold and turned into insulation boards. It has the highest R value of insulation boards and is resistant to water. It's performance goes down in very cold climates (google for more details).
XPS is extruded polystyrene insulation. It's supposed to work well when wet, very resistant to water, has decent R value, not as good as polyiso. However when colder or wetter it performs more consistently (I think).
EPS is expanded polystyrene insulation. (think packing popcorn). It is the cheapest foam insulation board and will give you the most R value for your buck. The broccoli bus insulated with lots of EPS and is happy with it. I don't have a lot to say about it aside from the fact that the broccoli bus is an outstanding build and makes me think better of EPS because it was used in it.
Batting:
I don't pay much mind to batting. I think with all the vibrations that occur in the bus on the road that stuff will succumb to gravity and sink to the bottom. Maybe not though, I'm just not into it. Except for maybe rockwool in select areas. Maybe.
Fiberglass batting: supposedly has good R value, and supposedly isn't harmful for you. It is waterproof but can still accumulate mold via dust/moisture.
rockwool: Pretty much fireproof, cool.
recycled newspaper insulation: molds easily.
These are opinions based off a lot of research.