Jeff, all the best with your new purchase, to keep it short you will need EHU (10m or 25m) if not supplied with van, a minimum of 85ah Battery, again if not already supplied with van, Fresh Water container and separate Waste Water container, plus a Gas Bottle (Red or Blue) and its relevant regulator.
Sometimes the private seller will leave the EHU as this should be part of the original contents, but will keep hold of the other mentioned goods, as they will/can be used on the next caravan, others will no doubt add to my list with things like crockery and cutlery etc but as I said I am trying to keep it short, as the first time you will want to relax and enjoy the experience and then decide what else you need/want, best regards Martin