We have a fairly comprehensive Accommodation in Greece page which I'd recommend you look at, if you haven't already.
It mentions which islands are the most expensive and popular, so you'd at least have some sense of the market. It also talks about finding accommodation, renting and buying.
Hope this helps and that you have a happy, sun-soaked retirement ahead of you.
All the best,
If you are looking for a greek island that hasn't become too commercialised, how about doing some reasearch on Symi or Ikaria. Symi is popular with wealthy expats and those that enjoy sailing, it is quite exclusive and doesn't have a huge tourist industry. Furthermore, there are regular connections to the island from Rhodes.
Another option would be Ikaria which is really untouched and you would need to be willing to learn a little of the local language to get by here.The vibe is said to be very laid back, with shops opening only at midday but staying open late into the night. The community is strong here so expect to get involved in local festivals and celebrations.
With so many Greek islands to choose from you will need to decide which suits your lifestyle best.