Do they understand your End-Vision?
It is extremely important that the Drupal development company completely understands your end-vision to be able to deliver exactly what you’re looking for. Do they have an agile and expert team to be able to manage all aspects of the development, starting from user experience design? When you choose more than one company to handle different segments of your solution, it can get pretty complicated as your end-vision might seem lost in the chaos. It might be best to go with a single provider with an end-to-end expertise. The team you choose should be well rounded and have an expertise in UI/UX design, Front-end implementation and Drupal development. Can they also provide Mobile app (iOS, Android or Hybrid) development and provide API access from various end points? Once your website is ready, you might decide that you need a mobile application and so you need to make sure they can take care of that too.