Bus windows are single-paned, so their insulating value is R-fuhgeddaboudit. Even double-paned RV windows aren't fantastic insulators, and even their hermetic seals will eventually fail from the road vibration. I've kept all my original bus windows in place and I'm building covering inserts for them out of 2" XPS foam board faced with 5mm plywood, held in place with snap-down draw latches, easily removable as needed. For fun I'm even going to trying building a few of these with double-paned glass salvaged from house windows embedded in them, to get at least a little light in side when I'm buttoned up in XPS.
Doing a DIY window swap in a bus is maybe a 2 on your scale, definitely doable and something from which you will develop skills that will be useful elsewhere on your project.