Hello Bobwilk,
Especially when buying a secondhand but newish caravan that might still have some manufacturers warranty left, it is vitally important for it to have a full up to date record of approved services, otherwise the manufacturer will not support the remaining warranty.
I get the impression that you are buying privately and not from a dealer, so you should be aware that whilst you might save 10 to 15% of a dealers price, you are on your own if anything is found untoward with the caravan, whereas with a dealer you will not only get a dealers guarantee, but you will also have the backing of the Consumer Rights Act which can be a very strong ally if the seller doesn't act fairly with you.
For most people buying a caravan is a major investment, and its only wise to take your time to get all the information absolutely clear and 100% satisfactory, before you make any kind of offer to buy. Unfortunately there are a increasing number of fraudsters out there.
Once you have identified a possible purchase, read up on either this forum (using the search facility) and try and find a brand owners website to discover typical issues the make or model has. Its really worth either taking an experienced caravanner with you to look at the caravan as they may spot things a novice might not, and its usually money well spent to have a Approved Workshop Scheme (AWS) engineer do a pre purchase condition inspection. If the seller shows any concern about allowing such an inspection, of the report contains any thing that your uncertain about, walk away.
Do CRIS check for ownership and any outstanding finance. Use your head not your heart.
There are some good deals out there, but you need your wits about you to avoid the dodgy ones.