You should not buy the theme if you already have a theme installed that you like. Also note that the design of the theme does not typically change to fit your website. I use the word “typically” because our css (plugin styling) is designed to be easiliy overridden and may be depending on your theme.
The cost depends on exactly what you need. For example, if once the plugins are installed, you need them resesigned to fit your theme, then that would have a cost. Simply installing the plugins for you is 1hr of product support