Clare,
I don't think it's uncommon I'm afraid.
http://www.eribaforum.co.uk/cgi-bin/topic_show.pl?pid=141866;hl=chassis%20rustNote, I don't say common, but not uncommon.
Common enough however, for a really good check to be vital along the lower small section frame member around the entire perimeter of the van.
To buy one with rusted 'small chassis' members would be a lot of heart-ache and money. I was quoted £1000's by the Scottish dealer for my problem. I eventually repaired it myself (that is to say, I 'managed' the repair) which was actually done at my local friendly engineering company.
if you do go to look at a prospective eriba, take someone with you and have a screwdriver and penknife each - two heads are far better than one in this situation.
Regards, Terry