You must dispay a valid tax disc in order to drive it, or keep it on a public road. Once you take ownership of it it is your responsibility to either tax it or declare it off road.
There are fixed penalties for failure to tax a vehicle and for failure to display a valid tax disc.
There is a section 10 on a Log Book - New Keeper Supplement - which you should be given when you buy the van. This will allow you to go into a Post Office along with the MOT and insurance certificate to tax the vehicle. You will then be able to drive the vehicle legally.