If you are looking for a van then you may have different requirements to that of if you were buying a car. Many people buy vans for work purposes to carry tools and materials and the amount you carry or the weight will help you decide what sized van you need.
If you only need to carry a small amount around with you then you may be able to by a car derived van such as an Astra van but if you need the extra height or side loading door then you may need to go for something like a Ford Transit Connect or even a Transit.
You should also consider the mileage, whether the van is petrol or diesel or if it has been looked after mechanically and also in terms of bodywork. You should try and check the suspension to check that the van hasn’t been carrying too much weight in the back.