Welcome to the forum Andy and best of luck with the maintenance jobs. Have a very close look at the small square section steel tubing that runs around the bottom edge of the van. If you see anything worse than a light surface rust you can rub off then seek advice from a caravan engineer.

Colin

Andy, where the skin on the outside is at the lowest level it is edged with a bit of aluminium trim. If you look underneath the van behind all this lot you will see what looks like a 20mm square section tube acting as a ring beam to the floor and structural steelwork under the van.
Eriba's are comparably good as regards damp but some do have rust issues and sometimes damp/delaminated flooring.
The metalwork will not have been galvanised, but will have been painted. This obviously wears with time and you usually get a very light rust on some of the steelwork, the sort where a light wire brush and repaint will put right. If there is any heavy pitting on any of the metalwork you will need to look closer. If you see any holes then walk away. It is a specialist repair and very expensive.
If you see any damp on the wallboards inside then likewise, walk away. You do not know what lies behind.
As regards patches of flooring that need replacement. This is a pain but a diy doable job and would require stripping out some of the vans built in furniture. Whilst doing this it would give you the opportunity to do any electrical updates you might want.

Colin