One of the many things that you have to think about is what car seat can you use?
The spica cast and brace put your little ones legs in a wide set position, which limits the kind of car seat you can use.
What we found helpful was renting a car seat before rushing out and buying one. This meant we had a safe seat to bring her home in and we could take our time to buy a seat.
The brand that was the most recommended to us was the Diono, because it had sides that can be adjusted. We also like the Luxii from baby factory which was a more affordable option - especially when you have two cars.