If you plan to move permanently, or semi-permanently, are over 50 and don’t plan to work in Thailand then it’s probably simpler in the long run to get a Non O ‘for retirement’.
It’s straightforward to get a visa and the an annual Extension of Stay so you could stay in Thailand indefinitely, or come and go as you please. If you are over 50 and don’t want to work in Thailand then it’s usually easier to get a Non O ‘for Retirement’ rather than one ‘for marriage’.
To start planning you should open a bank account in your name only the next time you are in Thailand so you can put the required funds when you need them.
Each office can have slightly different requirements so check what they want. eg bank letter, updated bank book, photos, address proof, passport copy etc etc.
I’m not sure what country you’re from but I found it very straightforward to get a Non O ‘for Retirement’ from the Thai Embassy in London using the E-Visa website. I then got the first annual extension in Thailand. I used an agent, even though I met all the requirements myself, as it’s worth it to me to avoid dealing with immigration.
If you go on a Tourist Visa then, yes, an agent can do everything for you. If you don’t have a bank account, don’t have the cash in a bank there and want a multiple Reentry permit then the first time will cost about
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baht. That will see you right for 15 months before the next Extension is due.
There are a number of things they might need but it can vary. You will need to check. eg TM30 (your friend will need to register you are staying there, if he is the owner), Residence Certificate, copies of passport, copies of rental agreement, etc etc.