Ironically "retirement" means absolutely nothing as you don't have to be retired in order to obtain the extention the Thai Immigration call retirement!
Anywhere. You'll get a better exchange rate in Thailand than you would in your home country or transferring the money. I agree about the visa requirements but the simplest way to overcome that is to get the Non Imm O in your home country before you leave, much easier! Then bring the 800k with you and bank it for the 1 year extention of stay. Job done!