Three or four people would be able to move the caravan ok.
One of the main places to check for damp / water ingress is along the interior wall / ceiling joint on the awning rail side of the caravan. Awnings constantly flex in the wind or when they get wet and this puts some considerable strain on the joint where the awning is attached to the caravan.
Another thing to look for is
cracked end panels, I'm not trying to put you off but it's best to know exactly what to look for as a buyer.
The overall condition of a used caravan is much more important than it's age, a well looked after bone dry caravan has to be worth a hundred times more than a flashy late model gin palace on wheels which hasn't been looked after and turns out to be a damp infested money pit. Before you buy have a really good look at what you're paying for, don't be frightened to lift cushions, to look behind curtains at the window surrounds. Gently ease the window rubbers back with a fingernail to look for black mould or moisture.
Take a torch to check underneath the sink cupboard for leaking pies and do the same in the bathroom. Check that all gas and electrical appliances work correctly (or as you mentioned, get an engineer to do it for you).
If everything checks out ok and especially if you're buying from a dealership try hard to negotiate a good price, sales staff often have a bit of wriggle room to make the sale so exploit it to the full.
No matter how sparkling and comfortable a prospective purchase first appears to be, don't get caught up in the excitement of the moment, concentrate on the negatives to the sales staff rather than doing their job for them.
Make them work for that sale!