Normally you’ll need to provide a copy of the original documentation used as the basis for income verification at the embassy. As last year's tax return or annual statement from your pension provider. Formally you’re not obligated to transfer or keep any funds in a Thai bank account as long as you’ve got a affidavit letter.
If you arrive on a 90 days Non O visa, you still should be able to open a bank account yourself although you might have to visit several branches. And you can move where you want after you’ve done your first extension. The main requirements between the immigrations is still quite equal.
It’s your choice, but if you’re passed 50 years and have income min £ 1500, you could just apply for a 90 days Non O visa. Then you’re excused from any onward ticket requirement, it’s cheaper in total than to apply for a tourist visa and do an extension and you don’t have to use (waste) time at the immigration in Thailand.