The age and mileage of a van, just like a car, is largely irrelevant. What matters is how it has been looked after and, crucially, how it has been driven. There is an inexplicable obsession that a low miles vehicle is better than high, and while that may sometimes be true, more often than not, it just isn't. Low mileage vehicles that are being used for short, stop start journeys during which they never get warmed up properly, are going to suffer much more wear and tear than one that has spent its life cruising up and down the motorway at their optimum rpm.
The main difficulty is trying to determine how the vehicle has been driven. It is easy enough to find evidence that it has been serviced regularly, but that is only half of the story. Unless you know the person selling it and how they drive, that is where the 'pot luck' comes into it!.
I have just been reading an issue of evo magazine, in which one of the 'fleet' contributors is detailing the latest work on a Lamborghini Murcielago which has done nearly 260k miles!.