There is no direct upgrade path from the website builder to MWP. However, you can leave your current site live, purchase and build out your WordPress site on MWP, and then point the domain to the new service once you're done.
The Ultimate MWP plan comes with a free SSL for the first year. Regular charges are applicable after that. It also comes with 30-day rolling backups, Website Security, and automatic updating of the core files.
* Please note that I offer free advice on this forum. I DO NOT PROVIDE ANSWERS VIA PM. If you would like personalized help, you can see my pricing on my services page. Otherwise, please ask your question in the proper forum so the answer can assist EVERYONE in the community and not just you. Thanks! *