I would like to create a Hosting Plan which allows to purchase applications only. When such a plan is purchased, the applications should be preinstalled .
There is no such option neither in Plesk nor in PBAS as preinstalled applications.
While creating the Service Plan in Plesk you can allow customer to install all the applications (by default) or to install only the particular applications: Service Plan > Applications.
The correct permissions should be set up for the Service Plan: the option 'Access to Application Catalog' should be enabled (On).
Also the User role of the customer should be checked: if the User role is Application user, the permissions for 'Install and manage applications' and 'Install and manage Java applications' should be set as 'Granted'.