Residence Permit for retiree in Poland

Can anyone please clarify the requirements for a temporary residence permit for a non-EU couple who wish to retire in Poland?
I have read that the permit is issued for people enrolled in a course of study, working, or starting a business, but also that "other circumstances" may be considered.
My wife and I will retire in about a year and would love to spend some years in Poland. We will have a stable income (Australian Pension and some investment income) and wish to buy an apartment in Warsaw or Poznan.
We don't have family ties in Poland, we will have  private health insurance and by the time we retire should have a very basic grasp of the language.
I would be very grateful for any information or hints as to where I might find it. Thanks

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