Like Mr Trooper, I'd recommend the Skoda Octavia. I've driven a few different diesel ones and they just get better and better...... only the vRS is probably going to be "fun", but the other ones will hit every button for you inc the 50 mpg goal.
Alfa 159 is nice (very nice)....... for that matter so would be a 147 JTD or even an Alfa GT JTDm.
The previous generation Mondeo in 2.2 ST TDCi format was deeply impressive, too. They will be getting a bit high mileage, now, but they seem quite tough and well built. Hertz used to run these and they were my favourite of the "ordinary" rental cars at the time (2006/7).