One major concern when converting a van into a livging / sleeping space, is ventilation. I did not want to end up with oxygen depleted air when keeping the windows shut due to rain or bugs.
A roof vent is ideal to solve these problems. I sucks the hot air out leaving you with a comfortable interior. My van came with a roof section designed to be cut out and have an air conditioning unit fitted which made it relatively easy to cut into the roof, fit the fan unit, and waterproof the whole assembly.