Ford popularised the affordable van with the Transit in the 1960s. Since then millions have been sold, and many have become camper vans. Still one of the most (if not the most) base vechicle to use for a camper van.
Ford Transits have proved themselves to be reliable, affordable and popular. Well know for their bulletproof engines, but they do sugger from surface rust, although later models are getting better.
There are lots of old models which can be had as bargains if you have the time and skills to give it some TLC.