Integrate Drupal with other systems using the web services API module. They can be used for reading feeds, exchange data between complex systems which generally produce Jason or XML files.
Drupal is an open source CMS perfect for SEO. It is the best SEO framework for large and complex websites as it is scalable, configurable and it has many useful modules such as Google analytics, url alias, meta keywords, page title, content types, custom views, search functionality.
If you cannot find a module that does exactly what you need then need to write a custom module. First try to find the same functionality in some existing contributed module. Sometimes it is wise to alter or extend an existing module rather then rewriting the functionality from scratch.
Create your own Theme from hundreds of existing themes and then customise it. This is a fantastic feature in Drupal to implement any kind of design in terms of look, feel and structure. Set up your regions such as the 12 grids responsive layout, Omega theme, very good for mobile.