It's very difficult to know what you're buying. Ask to see service records and blueprints, if possible, to determine where insulation is, and if the vehicle has been properly maintained. Costs associated with updating an old or existing unit can be prohibitive. Some groups that have done this feel they have invested more time, money and energy than they would have purchasing a new unit. Use the same considerations and requirements as you would for a new vehicle. One of the regular maintenance services with these vans is to regularly check and redo the roof to prevent leaks. If that wasn't done, then you may find an immediate need to redo the roof.
MHCN has a Registry of organizations interested in buying a pre-owned/used vehicle and organizations wanting to sell a vehicle. They carefully research the organizations and connect potential buyers and sellers, who then negotiate the details.