He said he was 45. Once he is 50 he can get a non-O visa as a retiree for 1,900 baht but I have met expats over 50 that have the Elite visa and are happy with it. I have a 10 year pensioner LTR.
I think you can carry as much as you want but you need to declare it at customs when it reaches a certain amount. I carried $9,900 USD into Thailand once and avoided the need to declare $10,000.
If you are asking about using the monthly income method to meet the 1 year extension financial requirements then you need to deposit at least 65,000 baht to your bank account each month. You have to show that the money came from outside Thailand (an international transfer).
If you are using the 800,000 baht deposit method then the whole 800,000 must be on deposit in your Thai bank account.
Why anyone would consider depositing exactly 800,000 baht to meet immigration requirements without having a substantial buffer on top of it to account for currency exchange rate fluctuations and mysterious unexpected banks fees is beyond my thinking.
If your US Social Security income is less than $25,000/year there is no tax. After that you would pay tax on 85% of SS. Exactly how much depends on your particular tax return. If you are talking about Social Security or the equivalent outside the United States then I don’t know.