First, Please get through the official installation guide for Discourse.
https://github.com/discourse/discourse/blob/master/docs/INSTALL-cloud.md
Then, There are plenty of good tutorials which explain how to set up nginx in front of discourse. Feel free to use the search button.
Running other websites on the same machine as Discourse
The last part is WordPress so You just go to the usual /var/www/html directory, wget the latest wordpress package and then extract it.
Install mariadb & php-fpm on the server and add a simple nginx server block that uses /var/www/html as root and forwards requests to php-fpm
Once that’s done, You should be able to access wordpress on your fqdn to complete set up.
This way, both the applications are installed. Now you can install the wp-discourse plugin to connect wp & discourse together.
If You need to clear any doubts before you begin, I’ll be happy to help.
Just so that You know, If you can reformat your server and get rid of plesk then You can save up the cost and get a working set up in about 2-3 hours if you have basic understanding of ssh.