partner & berlingo are the same van, you can get 600kg & 800kg payloads.
The new euro 4 versions are truely 600 & 800 payloads but the older euro 3 models are actualy 550 & 750 even though they are sold as 600 & 800.
connect & new caddy are good vans but are quite more expensive, the new fiat doblo 1.3 multijet SX is excellent value for money IMO & has a 740kg payload as standard but is available with a 850kg payload too.
With this size van you are looking at a 400ltr system & as mentioned before, fine for a one man set up but not two.
tony