Buying my first used caravan.....

Hi All. I'm hoping I haven't left it too late, but I am about to buy a used caravan in this next month. I am off to Silverstone for the F1 in July. I have never towed or owed a caravan before. Nerves are starting to set in. I've left it fairly late to purchase for a few reasons. Money being the first one, then having nowhere to store it currently another. Firgured leave till about now to buy one. My car is a VW Golf MK4 2003 hatch. It is the GT TDI model. Although this is a smallish car, been told should be fine towing at about 1200kg (so lightweight van). Car weighs about 1350kg. So trying to get near that 85% ratio. I have I guess about 4k to spend on a used one. I am concerned about buying one, as I don't want to be ripped off. I know ebay can be a dangerous place to purchase from, but since I need lightweight, my local caravan centre can be sparse with choice. I will try and test it out before using for the first time at Silverstone. I am really nervous though that I have left it this late, and perhaps so many things to get. Ideally the van may come with lots of extras. What advice would you give me please? I'm hoping to get a 4 berth. Many thanks. Paul