Do I need medical insurance and bank proof for a Non-O visa extension based on marriage in Thailand?

Ok, I re-read your question. Words have specific meaning in visa laws.

> I plan to get an O multi entry12 month visa, as I have a Thai wife from C.M. immigration.

You DO NOT want a Non-O multiple entry 12 month visa based on marriage to a Thai wife. You want an extension of stay based on marriage to a Thai national, along with a multiple entry re-entry permit. Huge difference in the requirements.

>Will I have to show proof of medical insurance.

Insurance of any type is not a requirement for an extension of stay or a multiple entry Non-O visa based on marriage.

>Also will i need a letter from my bank to show that I have 400,000 baht in my account

YES. Or, if your country still issues them, a letter from your consulate saying you make 40K THB a month, OR 12 months worth of foreign deposits of 40K THB a month, OR monthly income of 40K THB a month.

Generally the funds must be seasoned for at least 2 months before applying for the extension of stay based on marriage to a Thai national.

What do you really want to apply for?

Applying single entry Non-O based on marriage to Thai national from tourist status, then apply for 1 year extension with multiple re-entry permit based on same reason

OR

You are already on extension by some reason, you want to change the reason for 1 year extension to being married to Thai national

OR

You want real multiple entry Visa (sticker)? - this is not available inside Thailand as

@Benj****

says above, you need to go to Thai consulate in another country.

You can't get a multiple entry visa within Thailand