Custom Vans have several discrete systems and your build may have one, or all of these:
AdventureWagon Recreational Utility Vehicle (RUV) this is the base for all of our builds and includes all the interior walls, insulation, fans, basic 12v electrical, and L-Track
Suspension: Sprinters generally ride much better with at least new shocks, but that's just the beginning.
Electrical: If you want more than basic 12v you will need some type of electrical system which usually consists of batteries and an inverter and maybe some solar.
Water: Do you want to cook and clean with water from your van? If so you are going to need a water system. Water holding tank, and a pump at the minimum