If i bought a diesel, I think i would prefer a Fiesta. Its smaller, lighter, and wouldnt be as gimped as the focus would be. A 1.4 or 1.6 turbo diesel in a Fiesta would keep the fun, but double/triple the mileage.
I mean after all. If i had a diesel, It would be my commuter car, so i would look for something small, simple and cheap. I would still have a weekend car for fun.