I was married for 16 years. We had 2 kids. It ended in divorce as for her the grass was greener elsewhere. Nobody knows how a relationship will go, but I can say through experience it doesn't usually end well.
Wow! I have read many of the comments here and most of them are poorly written to say the least.
Why on earth would you want to have a teacher who has zero to basic command of written English. Therefore, I would ascertain that their spoken English is sub standard as well.
As stated above, you only need 400,000 baht for the marriage extension in the bank. Yes, there is a lot of paperwork, but it is easy to do. No need for any agent as you will need to collate all the paperwork anyway.