1 - Yes. Use no-ip.org so people can access your ftp with a domain name instead of IP.
2 - You can use any OS as long you can find a ftp server for it. I used to use Serv-U for windows.
3 - If you protect the FTP with password and only tell your friends you should have no problems.
Regarding the number of users some servers have the option to establish quotas for users. If a user downloads a certain amount of data in a day the ftp server will block that user for that day.
Good luck.