Todd Capron His visa status doesn't matter for buying a car. You can do it with visa exemption as long as you provide a residence certificate.

A residence certificate has nothing to do with your visa or permission to stay. It is solely linked to your address and TM30 registration. The required period of residence before it is issued may vary between immigration offices.

Ann Parris Estill You’ll normally need a residence certificate for to open a bank account, and it varies between immigrations what long-term living situation they require for issuing.

Immigration dependent, but you’ll normally need minimum a three months lease for the one year extension of stay application.

Alexey Shipulin I’m not sure which immigration office you are referring to, but at the main immigration office in Bangkok this has been removed from the regulation. If you look at the handouts provided at several offices, for example Jomtien, you will see the same.

This is now fairly well known and applies in several locations. Monthly transfers have also been removed from the regulation, and there have also been cases where embassy income affidavits are no longer accepted. That effectively leaves either 400,000 baht in a Thai bank account or income earned in Thailand.

The documentation requirements for a tourist visa vary between embassies and consulates. Some require little to no documentation, while others may request proof for the entire stay. As a "rule of thumb", a few days to up to a week of accommodation is usually sufficient. I would submit the hotel booking you already have in Chiang Mai and assume that will be adequate. If they should require more, you will normally be contacted by e-mail during the processing of your visa application anyway.

Yes just show both passports at immigration upon arrival and you’ll get stamped in on the new one.

John Holmberg Are you referring to applying for a new 90 days Non O based on marriage through the Thai embassy in Stockholm or a one year extension of stay based on marriage at the immigration in Thailand?

You only need to present a travel itinerary with potential flight numbers for the application; no documentation is required or needs to be uploaded.