I think the days of Fiat rust buckets have long gone - so long as you dont abuse the paintwork , but thats the same with any vehicle.
I think Mark made the initial enquiry about the Ducato as he serious about going truck mount and so not only needs some space but also gross and axle loadings.
Fiat, Citroen and Peugeot are sharing technology/costs both with the Scudo/Dispatch, Expert and Ducato/Relay/Boxer ranges. Same as Nissan/Renault/Vauxhall with the Primastar/Traffic/Vivaro range.