Hi CMB,
Well, quite simply... don't buy a Fiat or a TVR! (Ie. look for a decent car which doesn't have a reputation for breaking down!)
I, personally, would look at the car's current mileage. You don't want anything with, say, 100k already on the clock. Also, don't go for anything too juicy! I would go for a 1.8 max for fuel purposes.
There are many things to look for when buying a car, especially with that sort of mileage. You might want to have a sit in the car first too... you're going to be spending your life in the car and want it to be comfortable.
Also, don't forget to include a realistic business mileage onto your car insurance.
If it were me, doing that mileage, I would probably opt for a Focus Ghia. Take out some breakdown cover just in case too!
Good luck!