Any Shops offer Customizing Services?

I know there are companies out there that will provide custom vehicles at a huge price. But does anyone know of any company where say the customer could provide the vehicle( I had a Step Van or Box Van in mind) and the custom shop could build out the interior?

I figured that since the used truck and RV market is so huge, that I could save big money by buying used and then having a company finish out the vehicle.