Search ebay for Freedom caravans. You should get an old one in good nick for that price. Not much more choice in that budget as you don’t really want to be towing any more than 750kg for reasonable towing performance.
Otherwise Rapido folding caravans or you might find a pop top Flurette or Rapido. For any of the caravans mentioned you need to check them very carefully to ensure everything works & be prepared to do diy work on an ongoing basis.
You need to see running an old caravan as a hobby & ongoing project.