I would start with a few things :
1. Remove the seats so you have an empty bus.
2. Rust treat/insulate the floor.
3. Design your floor plan. Now that you have an empty bus, you can tape out things on the floor, make cardboard "walls" and mock up your designs. I think that's the best way to try and do things ONCE - plan, plan plan. Seeing the physical space will make it easier to do the layout.
You said you want to do a solar setup, so sit down, and start doing your calculations for how much power you need, what batteries you want, etc. etc. AC and batteries is going to require a hefty battery system. Lots of threads about that.
I would specifically hold off doing the insulation, walls, or ceiling yet - run your plumbing and wiring first.
It sounds hard, but there's about a weeks worth of work, then a lot of head scratching and pencil-pushing and planning before you jump in again. (At least, that's what I've seen 95% on the forum.)