You should probably enquire at uk.rec.caravanning also.

Damp and any water ingress is by far the worst you'll be looking at so
far as regards the living-in bit.  Obviously the chassis, running
gear, brakes etc need to be well in order.

I'd go buy a couple of the caravan magazines and read through the
'buyers guide' sections.

Chassis is easy to check, most are galvanised, so should be OK.

Damp in the walls is the main problem. It is fairly easy to check for with a
damp meter, but they would have to allow you to poke small holes in the
inside lining. They usually do so inside cupboards, or behind joining strips
so it is not visible.

Pieter