If you have never had a caravan before, then I would say, make a few decisions on the basic things, and then get something cheap, run it for a year, and by then you will have a really good idea what bits, you loved, hated, and could or could not live with.
First decision, how many Berths... and if you want the hassle of putting an awning up or not.
Fixed bed, or not, maybe go for a higher berth, where you can leave one end set up as a bed, when you want to, and still sit at the other end. 2 berth caravans tend to be better looked after than family ones!
Shower or not. Showers are nice, but on secondhand vans can be a source for major damp problems. Separate shower and toilet?
Carpets, or vinyl floor. Carpets are warmer, Vinyl, easer to clean, and you can put down mats.
Where you can keep it, and if there are any size limitations. Also if you do have restrictions on length, make sure you double check the "Shipping length", lots of people list the internal length...
Twin, or single axle, both have advantages and disadvantages.
As a first time tow'er, stick to the 85% rule, or less.
Heating? Hot water? Fridge?
Cooking, do you want an Oven.
Do you intend to only stay on camp sites with electric hook-up, or do you like the "hide away" sites. If hideaway, then the above need to be able to run off both gas, and 240v
Motor Mover?, if you have health issues, (and even if you don't) makes it a lot easier to level, and getin and out of the drive.
Things to stay away from...
Damp, if you go in, and there is even the faintest musky smell of damp, walk away...
Air fresheners, or smocked in... probably hiding damp smell. Walk away.
When you walk in the van, walk like you are bouncing on the moon, does the floor give, De-laminated... Walk away
Extra vents on the outside, in weird places... they are often used to hide dents and damage. Small dents can be expected, but punctures are very bad news.
Thing to look out for...
The caravan has been loved, feel in the cupboards, are they clean, or dirty, good owners wipe cupboards out.
Corners and edges, are they all marked up, or nice and crisp...
Manuals, do they have them all.
Do the internal lights all work, if they can't be bothered to fix them, what else have they left unfixed?
Cracks in sinks, and shower trays.
look under the cushions, are they as original, or have they been messed with, and possible been re-filled. In most caravans, real cushions, are sprung foam, and a lot firmer, than a lot of replacement plain foam filled ones.
Talk to the owners, ask them questions, do you get a good feeling, if not walk away...