What is the best free option for automatic ftp sync with finder? I was quite happy when I found out that Finder had native support, but it didn't take long until I realized it was read-only access. 

What is the best free option for automatic ftp sync with finder? I am using El Capitan. I'm not interested in very much functionality, just the ability to edit files.
TextWrangler

You can use BareBones' Text Wranger. It is a free texteditor with onboard ftp capability. TextWrangler is BBEdit's little brother, and BBEdit is one of the best text editors. I use this text editor at least since Mac OS 7.6 And, just like BBEdit, TextWrangler does not suck.