The service van must be at least a small cargo van. There is a large variety of models available from VW Transporter (as a minimum) to "tall" vans from Iveco, Ford, Mercedes, Renault, or Citroen. It can be bought used at a relative bargain price, but each used car requires service, various repairs, parts replacement as well as most often body and paint work. Than it must be also converted to fit all the required rally gear in a limited cargo space.
Experience shows that there is never to much space in a rally service van. The rule of thumb is: the bigger - the better. Most historic teams use tall vans. Full size trucks or even tractor trailers are typically not used in historic rallies. All depends on a budget, but Rally Service Van is a must. Its size and relative comfort is a matter of a budget. Low budget version of buying a used VW Transporter or similar, its mechanical and body service plus converting into a rally service van means spending of 5.000 Euro as a rock bottom minimum. There is no upper limit of what one can spend for a Rally Service Van.