Andrew,
We have a Berlingo 2.0HDi and also had a Partner 1.9D, they are both basically the same van, but the engine is what makes ALL the difference, the 1.9D engine is as flat as anything with very little torque, whereas the 2.0HDi stops at nothing, even fully loaded. We also run a Fiat Ducato and are looking at buying a Scudo in Jan.
The Fiats, Citroens and Peugeots are all basically the same, but the Fiats tend to be a good bit cheaper to buy that the other two. If you can afford it I would go for something along the size of the Scudo/Dispatch/Expert rather than the Berlingo/Partner as they have the benifit of a larger gross weight and also 3 seats in the front.