A community knowledge database website -- reposted

We are developing some utilities that we want to be able to offer to customers based on 1) Their operating environment i.e. Windows, Mac, Android, Linux, tablet, smartphone, laptop. And 2) Certain preferences that they select i.e. education, fun, children, etc. The website will detect what device/operating system the user has used to visit the website and will use this to determine what preferences they can select. Some of the preferences will be suitable for all platforms while some preferences will only be suitable for certain operating systems. An example of this would be where there is a game that will only run on an iPhone so once the OS is detected then the game category will only be available if the OS is an iPhone. Alternatively there may be a "how to change your screen background" pdf that is only relevant to a Windows laptop or desktop and as such should only be available if the OS is Windows 7 for example. This project will start off with this as basic requirement and once this has been achieved then the scope may grow depending on the success of the first stage. We have put together a spreadsheet with generic information that we would like stored in a database and anticipate that this would be in a MySQL database or similar free, lightweight database. Your proposal to us would indicate which database you would use and why. We have no preference for the code that runs the website but it must be capable of running on all platforms as above i.e. Windows, OSX, Android, Linux or any generally available consumer platform. HTML5 has been suggested for this. We would once again want some sort of reason why you have proposed how the website would be constructed and why. We see this as a community knowledge database that can be added to and grown by an administrator via an administrative interface based on feedback from the users of the website. This means that the administrative interface would also need to be developed. The website design component is not the most important component of this as we have a fair idea of what we want so the most important part is going to be the flow of the website as it detects the OS, queries the database and then presents the user with the categories that are available for that OS. Once this step has completed then depending on the selection that the user has made there may be a charge for the final service the user selects. This needs to be via a secure gateway that will accept any credit card or payment system. We welcome questions.