I've got a Peugeot expert 07' 1.6 3 men plus 650ltr tank ,pulls very well and good on fuel. 
I think that personal experience is the best advice.
I would personally go with the 2.0 liter engine as it has more torque and torque is what is important in a diesel engine, not horsepower. But it's much more expensive. Is the extra expense justified? We don't need to accelerate from 0 - 60 in 10 secs with 400 litres of water on board. My 2001 Citroen Relay with the 1.9 diesel non turbo engine was sluggish with 650litres but did the job just fine around the neighbourhood, although it didn't suit long motorway journeys. But the difference in time over 10 miles getting there was probably a single change of traffic lights at the end of the road - and the fuel consumption between the 2.0 hdi and 1.9 is very similar.
The C5 was given the 1.6 hdi 110hp engine from 2005 onwards. It was fine to be used in propelling that large body along. But forget towing a caravan with it. The earlier 2.0 hdi 110 hp was the best engine/gearbox combination for caravan towing - all because of torque.
My mate had one with his caravan. He changed the car (because of high mileage) for a Jag 2.0hdi with +-135hp and according to him didn't have a patch on the Citroen.
The 1.6 hdi will do the job as it is designed to pull the van with a load of 1200kgs, so will be fine for the job we require. But it wouldn't suit a 'boy racer'.
Personally, the most important things are remote central locking, electric windows and electrically operated/heated door mirrors, and a comfortable driving position. It also nice to have a good miles per gallon. I think the 1.6 has all those things.
One of the biggest problems we had in the motor trade was that we never got to drive a van with a full load, and they are totally different empty.