@Endre *******
your only problem would be, that you cannot open the required bank account on a touristic entry. You need to find an agent who has a deal with Immigration and a Thai bank, to allow you to apply for the "change of visa type" from a visa-exempt entry to a 90-days Non-Imm-O Retirement Visa, in the absence of a Thai bank account in your name. Some agents then will use the 2000.- THB fee receipt from Immigration to open a bank account in your name.