The first thing you are going to do is make holes in your van. You don’t want to put a nice ceiling in and then later on decide that you also want a fan, for which you need to remove part of the ceiling. Do you want windows, a fan, or a pop-up roof? Or maybe you need a hole for your electrical, plumbing or heating system? Think about how many holes you want to put into your van. Having windows helps to create natural light and if they open it will also create a nice breeze and help ventilate your van. A fan will help with that as well. However, having windows also means it will be harder to insulate your van later on. If you know you want to go to some extreme weather conditions: such as the desert of go skiing. Less windows and more insulation might be the better option for you.
For the Dutchies, if you are a little bit scared to put holes in your van like I am. Some specialised camper companies that might be able to help you with parts of your build: crazycampers, atrioncamperbouw, CMC hefdaken.