Hello,
Thank you for your purchase.
The Echo RSS plugin imports content from RSS feeds. By default, WordPress adds 8 posts to the RSS feeds it generates (this is why the plugin imported only 8 posts in your case).
To import all posts, you need to create a custom RSS feed on your source site, which will include all the posts from your original site. You can try creating the new feed using the Echo plugin or a free plugin, like this one: https://wordpress.org/plugins/custom-simple-rss/
After you created the custom feed, you can use it in the Echo RSS plugin settings on your new site to import all content using it.
Regarding full post content, please check this video for more info: https://www.youtube.com/watch?v=o-kGYmwwAZM
Regards,
Szabi – CodeRevolution.