Hi,
Without knowing more about your car it is hard to be accurate. We could do with knowing which exact model you have. For the rest of my answer I have presumed you have a Zafira 2.0dti CLUB, Yr 2000.
The Zafiras weight is APPRoX 1450kgs. 85% of this would be 1233Kgs, 90% would be 1305kgs.
As you are VERY new to caravanning both major clubs, and just about everyone else out there would suggest that you do not try to tow a caravan weighing more than 85% of your car. The lighter the caravan is the better. This means you need to look for a van with a MTPLM weight of no more than 1233kgs, or 1300kgs at the very most if you want to take a bit of a risk.
Any relatively new 6 berth caravan will be heavier than this, and most, if not all, will actually be heavier than your car, which I believe is illegal and VERY dangerous.