In reply to ogreville:
If you are buying second hand then consider the fact that many vans have a hard life and may need money spending on them, so consider who has driven the van before and how well serviced has it been? Also consider if the van has any aftermarket features, I've just bought an ex fleet LWB extra high Transit and it has a night heater, water heaters, the mother of all leisure batteries and did have a toilet, microwave, 3 water heaters etc that I'm in the process of ebaying (£100 for a second hand crapper!)
My brother in law was looking at ex police mobile speed camera vans and they tick a lot of boxes e.g. very well maintained, air con and heaters front and back etc...
Ex MOD vans would be worth looking at too - there are specialist auctions for them where you can see an RAC/AA check before you bid.
Other considerations are, are you wanting to fit swivel seats in the front, and if you do is their a bulkhead and is it removable? Also, look at past MOT's which will show up advisories.
Hope this helps, if I think of anything else to look for I'll post it up, but these are the first things that come to mind.