It all comes down to weight,engine type and aero dynamics at the end of the day.There is no secret formula,basically for the benefit of 4wheel drive there is a weight and shape penalty.
The only governing factor is personal choice,any advice is subjective and only you can decide what is best for you.
For economy, Peugot 106 Diesel.Simple as that,4x4- whatever takes your fancy but the Navara is bit less Juicey
Chuff me just seen your list of nice to haves,DO NOT get a Ford Ranger....they are shite