You need to go to the build thread section and do some reading. Start with build threads over 20 pages long. That's where the real info and experience can be found.
Asking questions will only get you opinions, not experience.
First off, the metal you use to replace the rusted metal should be at minimum 14 gauge. 12 gauge is what the factory used.
Take it up in small pieces. No easy way to go about it.
If your floor is that rusted, what does the bottom of your bus look like?
How about the U shaped supports? They usually fill with road crud and rust from the inside out. You won't even know till you hit them with a hammer from below and find they are nearly rusted through.
Hate to say it, but you made the same mistake I did. Never buy a rusty bus. To much money, time, and effort to try to fix it.
Pics also help.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization