Without a damp meter you can do several checks for any damage due to the dreaded wet stuff getting where it should not..
Hopefully the van has been closed so have a good smell inside. Can you detect any sponginess in the floor, particularly around the door. Look around windows for any discolouration of the wall boards, in particular around the large rear window where water ingress has been experienced by several owners from around the high level rear brake light. A check inside base cupboards would not go amiss. Rust has affected some vans, but usually in the same place. Have a look at the 20mm square tube that runs around the bottom outer edge of the van. This tube is surprisingly thin and if you have anything more than light surface rust it can be very expensive to repair, possibly making the van a write-off.

Other bits are pretty simple. Does everything work, turn on/off. Are the tyres in need of replacement due to age. Any dents and creases in the bodywork outside other than small stone dings are not easily repairable and will annoy you after time.
What extras/standard equipment does the van have. 12v battery and charger was usually standard on UK vans. Are the sockets 3 pin or Euro style. 5 stud wheels means you have the 1200kg axle whilst 4 stud offers only 1000kg and will limit what you can carry. Is a mover fitted. Does it have one onboard tank filled from outside with a hot water facility or just the two standard jerry cans under the sinks. Does the gas fire have the optional electric element.

Colin