"Her mother has given us some land up north where we build a house."
She has given her some land, foreigners can't own land.
"Retirement visa? I think might be good? Any pitfalls? Can you get it if you are married? My girlfriend is only 46, does that affect it?"
Put 800K thb in a Thai bank account. Her age has nothing to do with it.
"Marriage visa? I heard that I can do before we get married if we are engaged and it is cheaper? Do I have to adopt her kid? Is that possible?"
400K thb in a Thai bank account, Need to married, can't get it earlier. Youngest kid becomes your responsibility too when married, don't have to take any action.
"ED visa? Someone yesterday told me I don’t have to attend classes, can do it online."
Need to take classes and limited duration.