The Mercedes-Benz Sprinter and Ford Transit are both suitable for this type of use. I’d also add the Volkswagen Crafter and Iveco Daily to the list. A diesel should be the default choice if your son is planning to use the van for high mileage use. You’re right to be wary of diesels. The UK Government is considering a new tax on diesel fuel and major cities in Europe will probably charge drivers who don’t meet the latest EURO6 emission standards to enter. However, for large vans at least, there are not many non-diesel options. And if your son is planning to use it for mountain biking, he will probably not have to worry too much about city centre tariffs. Obviously, if your son is planning to spend money converting his van, he'll want to buy the best he can afford. That means low mileage, full service history and preferably one or two years old. For the top 10 large vans on sale right now, see: vans.honestjohn.co.uk/van-top-10s/top-10-large-use.../