I would just pain the rust areas with Hammerite, to save time and money.
Your installation ideas are all good. A plywood layer between will also help. Plywood is always the recommended sheet wood, as it is so forgiving in condensation and van scenarios.
Treat walls and floors the same really.
It’s fine to attach foam directly to the walls. You’ll need to glue them though, as tape wont be enough.
For the floor, you need to make a timber frame, with space about 1 or 2 feet wide. Then fill the spaces with installation. Then add a plywood sheet to the top to make the floor.
You don’t need a fan in the roof vent, although it does help. Yes, keep the venting hole.