Thing to realize is that you can put ply or whatever else you want over the bare metal interior but if you have an air gap between it and the metal you will still get condensation, only now its trapped behind the ply and will only be accessible for wiping off where it can run out. Condensation is formed when warm (moisture-laden) air meets a cold surface - the van interior metal. The only way to prevent that happening is to make sure that all the interior air-metal surfaces are covered with an insulator so that no warm-moist air meets the cold surface. Closed cell foam is perfect because it is light and 'warm' - open cell foam is rubbish because it allows air through, can still allow some condensation to form but now it traps the moisture and eventually gets mouldy and very very smelly.
If it was me, working on a budget, I'd find the cheapest closed cell foam I could and stick it on, forming it tightly around all the inner metal surfaces. Then stick polystyrene builders sheet insulation into the gaps between roof and wall strengthening ribs, and ply over the top with some thin carpet over the ply. Warm, dry, quiet, comfortable. Cheap.
If it was me, working on a budget, I'd find the cheapest closed cell foam I could and stick it on, forming it tightly around all the inner metal surfaces. Then stick polystyrene builders sheet insulation into the gaps between roof and wall strengthening ribs, and ply over the top with some thin carpet over the ply. Warm, dry, quiet, comfortable. Cheap.