I cannot comment on the licenses. This depends on the license arrangement you have with Oracle.
However I can tell you that you cannot configure Purchasing module without creating an inventory organization. Modules depend on each other as entities are shared among modules. This is very common. This create efficiency in the system as the data is stored at one location as oppose to various locations e.g. you can create employees through Payables module without marking HR module as installed. The data still goes in the same employee table owned by HR module. This is called Shared HR. Similarly you can still create an inventory organization in inventory module without formally configuring the Inventory module.

Purchasing module could be used without creating items in Inventory organization. In this case you would create PRs and POs on the basis of |item categories". GL accounts are associated directly with item categories. In this case you still need to create one dummy inventory organizations. But if you wish to use "items" in Purchasing then you need a proper inventory organizations (see Order Management videos).

Again I cannot comment on the impact on licensing. This is simple question that should be directed to your license manager in Oracle.