What type of visa should I select on the Thai Embassy website to apply for a Non-O visa, and how long do I have to enter Thailand?

Choice for 90 day single entry Non-O based on being over 50 (retirement)

Choice for 90 day single entry Non-O based on married to a thai/raising thai children

A Non-O visa is valid for 3 months from the date it's issued <- what that means is you have that amount of time to ENTER thailand when you do you will get stamped in for 90 days

you don't say WHY you want to get a Non-O visa; retirement or marriage to a thai.

Never the less you don't select TOURIST, you select Non-Immigrant

then either

- Retirement (pensioner aged 50 or above with state pension who wishes to stay in thailand for no longer than 90 days)

OR

- To stay with Thai family residing in Thailand (more than 60 days)