The downside of the non O is that you need to deposit 800,000 baht into a Thai bank account when you get here.
The sequence is:
Apply for the non O in your country before coming to Thailand.
Enter Thailand using the non o.
Open a bank account immediately (this should be possible with the non o visa).
Deposit 800,000 baht into your account in the first week or two.
Go to immigration and get a list of their requirements for a 12 month extension of stay based on retirement (every office is slightly different).
Wait until your money has been in your account for two months and then go to your immigration office with everything they ask for on their list, to enable you to get the extension and pay 1,900 baht.
You can repeat the final part every year. Good luck.
The idp is valid for a longer period than 3 months (I can't remember how long 12 month's?), but it is only valid in Thailand for 3 months at a time, so if you are planning to retire here, it becomes invalid after 3 months, as everyone is stating.
Up to you if you want to risk it, as long as you don't crash into me!