First, welcome to the site.
The intended use of your bus will answer some of your questions. Weekend trips, extended trips, fulltime in your bus?
Once you determine what the plan is, then some things become more or less important. If you are going to take short, fair weather trips, then you can skimp on insulation...(you'll want some just to quiet the beast down while driving.)
Most of us strip the interior down to bare metal because we know that most school buses have rust damage hidden under the wood/rubber floors. No point in spending time and effort building over a crappy foundation. Plus, once it's built, it would be significantly more work to have to later disassemble part to repair a rust hole in the floor. (Rust holes will only get worse) So stripping the floor down gives you a chance to remediate rust damage or if no rust, peace of mind. Then even a thin layer of insulation on the floor will make a big difference in road noise.
So, figure out the plan for the bus and have at it! Wishing you success!
(Looks like Caddy-kid is a faster typer than me)