Thanks for asking and choosing X and Pro. Welcome to X and Pro community.
As there multiple question, let me address one at a time:
1) What type of extension to install depends on your requirements. Here is the list of premium plugins that we offer to our customer for free and depending upon the requirement you can install. https://theme.co/x/extensions/
You can also take a look at following article https://www.sourcewp.com/must-have-wordpress-plugins/
2) Visual composer: Not necessarily. We have inhouse page builder tools for X Theme, Cornerstone. It's easy, fun to use and has sleek and powerful features that will help you to build engaging user interfaces. Pro comes with it's own page builder tool.
I am sharing link to our knowledgebase that I would suggest you to invest some time going through articles and videos: