Why must it be diesel? Unless she's going to do a shedload of miles, the extra purchase costs won't be recouped in fuel savings.

There may also be higher maintenance costs on a diesel, and with all the company car drivers going for diesel to keep the CO2 figure down, there could be such a glut coming to the secondhand market that residual values could collapse.

But if she rally wants one, there are a few available: Citreon Xsara, Honda Jazz, Skoda Octavia, VW Beetle are a few off the top of my head.