Ideas for starters

Our son and family have decided to get a small caravan, folding camper or even a folding caravan. They've done a lot of tent camping, now with just the three to fit in they're looking for a cheap secondhand unit that will give them some good holidays. Is it a good idea to choose a folding camper or would a small caravan be better? They don't have much storage space and they want something they will use for a few years.