Both the regular 90 days Non O visa and the one year Non O-A visa require min 50 years of age and monthly income min equal to 65K baht or funds equal to 800K baht.
The Non O-A visa also requires a mandatory health insurance, health certificate and a police check up.
The regular Non O visa gives you up to 90 days in Thailand before you’ll need to apply for yearly extensions of stay in Thailand more or less on the same conditions as the initial 90 days Non O visa.
The one year Non O-A visa gives you up to two years in Thailand if you update your health insurance also for the second year.
After this you’ll need to apply for yearly extensions of stay as for a regular Non O visa BUT you also maintain the mandatory Thai health insurance requirement for all future.
You can apply for a Non O visa based on retirement in country on a TM.87 for visa exemption. You’ll need minimum 15-21 days left on your current stamp. The requirements are the same for both entries.
If you’re from one of the 93 nations entitled to visa exemption you’ll will as a general rule receive a 60 days free stamp upon arrival as you did last month until the day the immigration officer you’re standing in front of no longer consider you as a genuine tourist to Thailand.
You apply on a TM.7 form at the immigration. Normally you’ll need a receipt of your TM.30 address registration privat or hotell. You’ll need a pass photo, copy if ID-page and latest stamp in your passport and 1900 baht in cash.
Yes you can just leave 24K or 12K USD (retirement/marriage) in a Thai bank as a guarantee and "forget" it, and that’s it.
But you don’t have to save anything if you don’t want to. The requirements are just minimum income for life expenses every month. (From the second year).