What is the best free FTP server for Mac OS X Server?
Unless you’re planning on running an extremely heavy loaded FTP server (which the mini won’t be able to handle anyway) the built in FTP Server should be enough. 

Pure-ftpd is a classic in the Unix world and I’m sure you can of course make that run under OS X, but it doesn’t have a “UI”, it’s all command line. (But very easy to configure anyway).