I'm absolutely no expert on cars or vans but we did a 6-month stint in a campervan (from Melbourne up past the Daintrees and back).
We bought a Fiat Scudo, about 5 years old for $17,750 - already fitted out by the previous owner.
We sold it at the end of the trip for $17,500, with the only maintenance during our ownership being 2 new tyres, a new battery for the fridge and the roadworthy when we sold - total ~$1,000.
So our strategy was basically to buy well, take good care of it, and then sell for a good price, effectively bringing our transport/lodging expenses for the 6 months down to ~$1,250 (not including gas).
We were with 2 toddlers and a dog in the van by the way.