Webbmac, I also already asked this question to Alan but the answer was : it will be more expensive than a build in an empty van. The most expensive in building an interior is hours of labor, not the fournitures. So, uninstall the complete interior, remove the walls to install better insulation and then reinstall fournitures + change some of those, it's a lot of work which would be more expensive than install a new interior...
Thanks for keeping your eyes open for my van!
Of course I will keep the forum updated of my project and my trip... but before, I've to start it concritely by finding the right van!