1. FileMaker Advanced offers no special features to specifically support synching data. Both FileMaker Pro and FileMaker Advanced can host a database so that a limited number of simultaneous clients--including FM GO clients can access the database so that synching is not required. I recommend that you visit the web sites for 360Words and SeedCode to investigate the synching tools that they offer if your FM GO users will be collecting data at remote locations where wireless access is not reliable or fast enough to work well.
2. Whoever is going to host the database needs to buy the license. There are also hosting services that, for a monthly fee, will host the DB for you. You would not specifically need to buy a new copy of server for each project as one copy of server can host multiple files for different users. There is, however, an upper limit to how much "traffic" a specific server can support before you experience undesirable delays.
3. With Filemaker 13, there is no longer a FileMaker Server Advanced option. FileMaker Server now is licensed under a new model and supports a new method of web publishing called Web Direct: http://help.filemaker.com/app/answers/detail/a_id/13102
That said, I strongly recommend that you purchase FileMaker Advanced if you are going to do anything beyond the development of very, very simple database file. The developer tools that come with FileMaker Advanced save you huge amounts of time and frustration, help you better understand how FileMaker works (nothing like watching a script execute one step at a time while seeing the data in fields and variables change to better understand what's happening), and provide additional options for how you design your user interface and calculate values.
Thank you very much for the response.
Do I understand correctly that, provided wireless connection or 3G is available, the users access the main database directly from Filemaker Go so that changes they make on the mobile devices update the main database?
Your answer has been really useful, thank you, I will indeed choose Filemaker Advanced.
Yes, but you have to correctly set up access to the hosted database (Port forwarding is one part of that) and the response times--especially for 3G may not be fast enough to be practical, you'll have to test things and see how they work for you . To avoid the risk of purchasing server and then not needing it, you might pay a hosting company to host the DB for a month or two and then use them as your test bed to see how well it works for you in the actual locations where you need this to work. You may find that some of the areas where you use this have "dead zones" where you can't get wireless access or only very slow access to the database. How much data you have to send back and forth from the client device will also make a big difference in your response times.
There are three Scoop.It topics about Filemaker (one is mine) they may be worth a look. They have multiple pages - navigation is at the bottom.
Monthly cost Licensing Filemaker Server $29 (1 connection) $59 (5 connections)
To host or not to host
Thank you David, I really appreciate this. I will take a look and (perhaps) come back with a question or two if that is ok.