I have been in the process of building a simple frontend for Magento, but stopped because the API only allow access to products, categories, orders and customers, and I would at least also need Brands. Is there a plugin or other way I can get the information I need via a REST endpoint? If I upgrade to Magento 2, would that help?
You can customize a native API REST of Magento with third-party module: https://medium.com/@denniswalters84/how-to-extend-and-customize-magento-rest-api-a842d0071682
When you upgrade to Magento 2, you should rewrite your module.