Hi
@Jules , do you still have questions on this or did you get it sorted?
You might have the Roof Rack preparation in the gutters of a Caravelle which would make a big difference to the roof rack required. It looks like a grey/black slot track bolted into the gutters on the roof of the van.
I am also a windsurfer and a bit of a roof rack nerd. I just bought a Thule roof bars set from Amazon for my LWB with the roof rack prep.
If it has the prep you would need the 135cm Thule wing bar evo load bars, (other Thule bars do fit but these are Aero). A Fixpoint Fitting Kit part number 3057 and the Thule 751 Fixpoint Xt Rapid System Footpack for cars with built in fixpoints.
I bought all this on Amazon for about £179 all in, hope that helps.