It is a simple (though sometimes tedious) process. But since you're already convinced, not sure what you're asking about. Use an agent if you want, but use your own money in the bank so you can remove your lips from the agent teat in the future.
I left my Fi data off. Butt then my phone died so now just use Fi web for sms. They won't ship a replacement phone out of the country (not very supportive for a company that offers international service). My Moto Razr died 2 weeks after warranty.
Good be much better if doing for the non-o via the evisa system before you arrive. You'll then have 30 days to open a bank account and deposit the 800k. Much easier process than applying after you arrive.
You be better served leaving and applying for your non-o outside Thailand. You might want to consider being on retirement of you're old enough. Marriage has a lot more paperwork and it's a hassle every year.