Full version of DigiShop includes PayPal payment gateway. You can download a free lite version of DigiShop here. It has the same features as the full version but doesn't implement PayPal payments, that is why only free downloads can be served. However you will get familiar with DigiShop options and get idea of administration and frontend application flow.
If you already have DigiShop Lite installed and wish to upgrade to full version, purchase it here and install it without uninstalling lite version, it will not overwrite current component settings. Naturally, you will have to add configuration for your PayPal seller account (lite version doesn't support payments and account settings are hidden in component configuration).
DigiShop plugin for K2 adds support of K2 items in DigiShop. It will work with both full and lite versions of DigiShop. Please, note that if you are using DigiShop Lite and try to setup up a paid download in K2 item, you will get a fatal server error in frontend when K2 item is viewed. Refer to this page for details.