Road from one.com website builder to proper CMS

I have "inherited" a smaller website hosted on one.com. Currently made using the "website builder". It's a horrible mess of flying frames and boxes that can be moved and placed anywhere. I feel very much back to MS Frontpage.

I would like to use a proper CMS on the site. What is the process to make the "front page" eventually be the CMS instead of the website builder, and is it adviseable to use one.com "wordpress" or better an alternative CMS ?

Anyone have actual experience moving away from website builder ?