Years ago I use to buy 2nd had vans for my Truckmounts ... Though when you do a proper install (Insulation ply and speed lining - great graphics and so on ) you would like to keep the van as long as possible - Yes it is true you lose money on day 1 with a new van but you easily offset this against the extra 3 - 4 years you can keep the van longer and the resale value keeps better
Depends on your truckmount but I have found Citroen relays great vans - I get my vans form Citroen in Cambridge
Chris at Alltec does a great job with the installation
With a new citroen Relay you can opt for a rear suspension upgrade for very little money and you can get a few more tailored extras like Air-con and your preferred color, reversing sensors / Camera speed limiter
If you need more info I would suggest you contact Angus from Citroen on 07770331154