If you're small, there'll be cars in which you can't reach the pedals or the driver's seat won't have enough height adjustment. Choose a make with a reputation for reliability. Most modern cars are tough, but there are still some with reliability glitches. If you're in doubt, look at the JD Power Car Satisfaction Survey or the motoring magazines. Check the car is crash-resistant in an accident, again most motoring magazines have crash data.
Flexibility is important, too.
Make sure your car fits you. People with lots to carry need a people-carrier or a capacious estate. Don't buy something that's going to let you down every other journey. Safety's big business. Get as much as you can.