Hey there,
Depending if you need basic or full RTL support in epesi, it'll require little to huge amount of work.
To get the full effect, you should change Base_Box default.css file and modify body style, add direction:rtl. This will make quite a mess of epesi, since many templates were not prepared with this switch in mind. You'll then have to manually go through all the template files, css files (in every module, this will require good knowledge of epesi structure) and images and alter them to get desired effect.
The other option to is to handpick the important parts (classes/IDs) you want to switch direction to RTL and simply add them to any css that is always loaded (Base Box default.css is a good pick). This will leave some imperfections, but will require much less work than the first solution.
Good luck,
Arek