You're likely to need a suitably sized van for carrying ladders, tools, equipment and sundries to and from jobs. You'll probably need a good roof-rack for carrying your ladders.
You might want to fit a shelving or racking system on the inside of the van so that your tools and sundries are stored in a neat and orderly way and the van becomes a mobile workshop. Be wary though about leaving anything of value in your van overnight on a regular basis - it's likely to get stolen. You might decide to upgrade locks and fit other security features, such as an alarm and strong-box. Nevertheless, your insurer may refuse to cover tools and equipment left in your van overnight.
The older the vehicle the less it will cost to buy, but very old vehicles may not be economical to run and may project the wrong image for your business. An alternative to purchasing a new or nearly new vehicle outright would be to lease one.
If you're buying your business vehicle, think about the likely resale price as well as the purchase price. Some makes and models depreciate faster than others, so although they may be cheaper to buy they might cost you more in the long term. Think about things like day to day running costs, fuel consumption and reliability too.
Consider having your van sign-written to advertise your business and services. Although this can reduce the resale value of the vehicle, it can be a really effective way of advertising. It can also help to project the right image. Professionally designed self-adhesive cut vinyl lettering is available as an alternative to traditional sign-writing and can be removed before the van is sold (although it does often leave a visible fade mark). Vinyl car body 'wraps' are also available to cover part or all of a vehicle in brightly-coloured graphics.
Make sure that your vehicle insurance company knows that the vehicle will be driven for business purposes, possibly by several different drivers.