You do not need anything to apply for the 90 Day O, you are not a resident yet. You will need it to do your 90 day reports.
To apply for the one year extension of the O, it will be easier with a lease and matching TM30. That is one of the things they give you the 90 days to arrange. A letter from friends or family inviting you to stay, along with a copy of the housebook would work.
The currency exchange house needs to be outside Thailand to show the international transfer. I would go UK bank to Thai bank to make it as clear as possible.
You will need a new TM30 to file your TM47 90 day report at your new address. When traveling in country, book in your tiraks name to avoid new TM30 filings.
400k or 40k a month. You can use a certified pension letter from your embassy if you can get one, or you have to show twelve consecutive months of international transfers.