Hi and welcome to the forum.
Buying a caravan for the first time is big step, and you must use your head not your heart when choosing!
With your quite restricted budget, you will definitely be looking at fairly old caravans, and it's sad fact that all caravans do deteriorated with use and age, so you do need to know what to look out for, and as novices to the hobby, there are some things ready to trip you up.
You already received a number posts with some good advice.
When looking at second hand caravans, it can be great help to take someone along with you who is already a seasoned caravanner, as they may spot particular points that you as a novice might miss. Do you have a family member or trusted friend who caravans? Most caravan hobbyists would be proud to be asked to help with your quest.
Dealer or private sale. This is often asked because on the face of it, a private sale seems to be 15 to 20% cheaper than a dealer, BUT, when you but privately you generally have no back up if things start to go wrong, but by using a dealer, you will probably get a limited warranty from the dealer, but more importantly you will also automatically get the protection of the Consumer Rites Act, which applies to S/H and New sales. and this is a very important point , it's worth researching for general information.
A dealer is expected to use their expertise to help the customer make a good choice.
A full service history with all the up to date tickets stamped and signed, and carried out according to the manufactures schedule is a good sign, and hopefully will show previous owners have taken care, and more likely that any repairs have been done professionally.
Prepare a written list of "open" questions, for example :
Don't ask "Is the caravan damp?" preferably ask "Has the caravan have any history of damp?" You need to get the seller talking about the details, not just a Yes ,No opportunity. What the seller tells you is considered in law to form part of the items description, Keep a note of any such answers. It really helps if a some one else is also present to remember all the key descriptions you have been given.
Most importantly, if anything does not add up between what you see and what you are told, use your head and wonder what the seller may be hiding. If you are not 100% satisfied be prepared to walk away.
Edited for clarity