As you are changing cars, buying an older vehicle may be cheaper. However, older vehicles consume more fuel, so you should take a note on this. The newer a car is, the more efficiently it works.
The engine also makes a difference. Bigger engines burn more fuel during the same journey as a car with a smaller engine.
Car weight is another factor which affects fuel efficiency. Heavier vehicles need more power to run than lighter ones. And of course, remember that every car needs to be manufactured when energy is spent in the process, and CO2 is released.