We'd recommend a Honda Jazz. It's a really versatile small car with a low loading lip that makes it ideal for transporting dogs. The latest model is a hybrid that'll be very cheap to run. Alternatively, take a look at a Skoda Fabia Estate. The latest model isn't available as an estate but you can get a good deal on a late example of the previous model. It's a compact estate car (not much bigger than your Swift) with easy access.