The cheapest way to drive a van is to buy it new and drive it for 9-12 years, divide the monthly costs over the 9/12 years and it's a no brainer plus you have the benefit of the van not having been abused and should run well.
If your vat reged then it's even better, buy it at the end of your quarter or year and you claim that back right away, which is normally your deposit. As Dan says I've been advised that it's a 3 year costing unless you lease, but then your heavily constrained by mileage and condition.
Ultimately there's no set way that's right for everyone, it depends on your business position at the time of purchase. Wether your cash rich for deposits, vat reged etc, the glove needs to fit the hand. One thing that can't be underestimated is image, to some builders you'll look like you charge too much and to some customers you could appear as if your not good enough to be able to afford a decent van. Horses for courses!
