From a tourist visa or visa exempt entry you can go to immigration in Thailand and with 800k in your Thai bank on day of application you can obtain a non O retirement.
That visa gives a 90 day stamp.
In the last 30 days apply for 12 month extension.
Regarding use of agent.
Depends what you require from agent.
What is your location.
Edit: just read back in thread. You are in Bangkok.
You could discuss your options with ThaivisaCenter.
In Sukhumvit you could chat with Bangkok Buddy (Tanya)
If you are providing your own funds you can do it yourself.
Not difficult.