I love tools like Code Canyon as they bring more businesses to Flutter and get Applications off to a good starting UI. However, you should always check the app description, demo, and customer feedback before buying any app template from CodeCanyon or any other source.
You will need a developer experienced with flutter to customize the business logic and connect any back end services you have. I recommend asking a developer to review a package before you purchase and then see how much you are able to customize on your own, or with the seller, before reengaging the developer again. This will help save on costs and provide the bet chance at a successful outcome.
Flutter receives regular updates, always check the package is on the most recent version and receives regular updates.