In reply to AJM:
This is based purely on what I've driven (I've done a lot of miles over the years in work vans), not for their convertability, but I'd go for a Sprinter. I find they are reliable and the nicest to drive. Transits are perfectly OK, but I find the drive less appealing than the Merc. The Vauxhall is completely fine, but not inspiring in any way, whilst I won't touch a Renault as I've had too many mechanical issues over the years.
I'm sure others will totally disagree, but there you go!
I can also recommend my own vehicle - a T4 syncro