If its the last series (not the latest) then the 1.9d has an 815kg payload and the hdi has a 900kg payload.
You have to consider everything you put into the van as part of the payload, that includes, fuel and you the driver and anyone else, ladders and roof rack/bars. A 650 liter tank in a 1.9 will be an issue weight wise, so the hdi/jtd would be a better bet for this size tank, although you will still have to be careful you carry only what you will need that day.
As was said, you will be ok with 500 liter provided you don't load it up with other things - ie carpet cleaners etc.
Spruce