First, you should note that Magento web development is not a short-term game. It involves a long process of maintenance and constant updates. So, make sure to hire Magento development company that can take care of your every requirement including development, deployment, support, and maintenance.
Also, choose a firm that has prior experience in building Magento websites. Make sure to talk directly with the development experts so that you know they have understood what they have to do for your business.