Signwriting your van is a great form of advertising. If you have a website, be sure to show it on your van. Striking graphics will get you noticed and your name known in the area.
A sensible investment is to protect your van from corrosion. It involves covering the vehicle floor with a tough, water proof coating. If you are carrying water in the back of the van things are going to get wet and hence will lead to rusting. This coating will protect the resale value of the vehicle.
The right van for your business is an important decision. It will obviously depend on your needs. Will you have a van mount system or will you be carrying a trailer? If you intend to use a van mount system, what size water tank will you be carrying? Are the measurements compatible? If you live in a busy city, would a smaller van be easier to park?
Remember if you are carrying large amounts of water on board then you will need to take particular care that the payload of the vehicle is sufficient for the weight that you will have on board. A litre of water is one kilogram, but you need to take into consideration additional weight of passengers, fuel and tools.