With the goals and reasons stated, I think you need to look into making your own biodiesel or converting to WVO. The other option is a 190D 2.2 with a 5 speed which seems to be reliable for 40MPG on the highway. Also, my 240D manual gives a steady 28 MPG in the city. I wish I could tell you more about the 190D but mine is still on a transport truck that is meandering through the southeast according to the handy dandy tracking function provided by the shipper.

Kevin
240D
190D