A lot depends on your personal circumstances, are you vat registered? which means if you buy the van at the right time of the month you put the vat down as a deposit and claim it back more or less right away, which lets you drip the balance over four or five years. dealers offer very competitive rates and at the right time of the year you may even get 0% APR.
If your not vat registered then pre reg or ex hire with a long warranty would be the way to go. you need to make a decision based on your own personal cash position and whether or not you can afford to loose the deposit out of your cash flow.
Leasing companies have it down to a tee, the cost of contract hire is more or less the same as what you loose in deprecation, but on a three year cycle the deposits you have to put down mean that it works out more expensive than buying new, there's a big market out there for people who can't afford the bigger deposit of buying new.
Personally I think the benefits of a new van out way the uncertainty of a second hand van that you don't know what it's been out through. Apart from having a much more professional image, (which if your after the right type of customer they don't mind paying for quality
) you also have an expense to claim for in your business.
Imo if you can afford the initial outlay of the deposit, over the course of five years and from what you can claim back, the reliability and new 'good' business that a new van attracts makes it a no brainer and I think in the long run it makes you money.