Get the non O 90 day visa online from the Thai Embassy before travelling to Thailand is the simplest way. You can then open a bank account in Thailand and extend your stay by 12 months annually.

It used to be possible to open a bank account in Thailand without a long term visa until this year, when the banks were scrutinised and stopped. He therefore needs to travel to one of the neighbouring countries ((Vietnam, Laos etc), apply online for a 90 day non-O visa and once granted, return to Thailand, open a bank account to start the process.

When did you open your bank account without a long term visa? Was it this year? Did you use an agent?

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?