Get your weights right, what are you going to use for a tow car ?
Look for a plate, usually near the door of the caravan, you'll find an ex works weight and hopefully a figure called MTPLM, this is basically the maximum weight of the caravan and all the associated junk you carry in it. The recommendation is this is 85% or less than the weight of the tow vehicle - this is law and you can tow anything you like but it would be sensible to stick close to the 85%.
Far as the caravan itself goes look for any evidence of damp inside cupboards, in the under seat bunks - anywhere you can get to.
Even at the age of van you are looking at the van will have a hot water system (powered by 240v mains or gas), its very handy to have and the system are relatively sensible.
Have a look around underneath it, check if the corner steadies are lubricated - a sign that someones looked after the van.
If you find one with an awning its a bonus, but don't turn down one without - have a hunt around local dealers, typically when people trade in their vans they find the awning doesn't fit the new one and quite a few deals will do part-ex and then sell the old awning on. Even new they aren't that expensive any more. Only bother with a sectional awning where the front/back/sides zip out, make it so much easier to deal with and if necessary dry out.
There is an optional caravan register, you get a document which is sort of the V5 for the van : http://www.hpicheck.com/caravans.html
If the owner can produce one its a good sign.
Hitches & stabilisers.. old caravans had a simple ball hitch that required greasing, you then had a separate unit for a stabiliser, something like : http://www.caravangroup.co.uk/admin1/image/7371400...
The stabiliser stops side to side movement of the caravan. Don't have to have one but there is no reason why not really.
Lately new hitchs have been developed that have a built in stabiliser - the van you linked to has one. There are a couple of different versions, but basically when the ball goes in to the hitch there are little pads that push on to the ball very firmly. Much neater solution and are very effective - it is very important however that you do not grease the towball on the car at all, otherwise you negate the grip the pads have.
Look at the caravan club, I think they do a pack of general information on starting caravanning and even run courses to help you get used to driving and manoeuvring with the caravans hooked up.