Yes, you need bank statements, updated bank book and a letter.

Yes the non-OA visa is available from your home country and it's a 12 month visa.

Yes, the 800,000 baht is to show you have the money to pay for a medical emergency, without that you need insurance. It's pretty logical.

If you get the non-O before you come you have done the difficult part already. Why not try to do it yourself first before engaging an agent? Opening a bank account with the non-O should be simple but might involve asking at a few banks. Extending your stay is one A4 form with about 15 questions (name, address, passport number, date arrived in Thailand etc). You need photocopies of passport bank info, but it's very simple.

I don't really understand your first question, if you supply all the information I am confident you will get the 90 day non--O visa. If not you can enter Thailand visa exempt for 60 days unless you have a history of coming to Thailand without visas.

2. Yes 3 months statements should be enough for the financial requirements (see picture).

If you don't want to deposit 800k, but want to stay long term in Thailand, why didn't you get the non-O-A visa instead?

Yes, when you enter Thailand on the non-O visa, you need to open an account in Thailand and deposit 800,000 baht in the first week. You then wait 2 months and apply for a 12 month extension of stay.

Yes, online at the Thai Embassy in Aus. It's a bit confusing as it's a 90 day visa based on retirement and the wording is a bit strange. It looks like this...