Chances are that converting your files to 10 will not be a flawless process and will need at least some repair by a filemaker developer.
I've converted over 50 5.5 files to 10 and while the process was fairly successful, I still had to patch a number of things here and there before I had the new version up and running.
(Once you do that, you'll find that there are a goodly number of improvements you can make to your original files that were impossible in your original version, but that's a whole other thread.)
Additionally, you may find you have to upgrade your server hardware and OS in order to install fmp server. (Check the knowledge base for system requirements for a more complete picture than you'll find in the sales info.)
Keep in mind that sooner or later you will have to upgrade your system or you will be unable to acquire new computers to replace your current machines when they arrive with operating system versions that are incompatible with the older filemaker versions.