I finally managed to create a subscription plan and a subscription. The invoice got created automatically. YAY… (perhaps I should have tried the Auto-Repeat feature first… but now the subscription ist working).
But I need help with this last step: The customer does not know about the new invoice, because it got not mailed automatically.
I tried to set up a new Notification for this, but I only want to fire this, if the invoice is auto generated by a subscription - how can I do this?