One central license of Server. Host all files on that server.
If you're trying to use a desktop PC as a server, you're going to have issues. They just don't have the juice (not to mention the OS features) to perform well as servers. If cost is a concern, then perhaps a hosting service would work for you. This is especially true if your users want to access the files "from anywhere" (i.e., over the open Internet).
Nevertheless, using FileMaker Server is the most appropriate route for a multi-user scenario. You get automated, managed backups, good control over user sessions, logging and metrics to isolate problems, and much better security options.
You would get one server to host all the files. Only one central server.
The Team clients need to be connected to a FM15 Server to open local files. Essentially the same as traditional licensing but with the requirement to be connected to a server at all times.