we have a Isuzu Mu X the insurance is about 20,000 baht per year and another 3,000 baht for license.As far as sitting for 5 or 6 months that ha s never caused a problem we also have a Mercedes in the states that sits for 5 or 6 months and again never a problem.
I have been living in Thailand 5 or 6 months a year for 20 years,with my Thai wife, we bought a car the first week we arrived in Thailand 20 years ago. We are now on our second car I would no way be without a car in Thailand. Just me but I would feel completely trapped
One thing you need to learn if you plan on staying in Thailand long term you need to understand above all saving face for a Thai is most important even if you were correct the first time you were at immigration you will not convince them. Make life easy go to a neighboring contra get a 90 day non o based on marriage then get all of your paper work together and with 30 days left on your visa apply for one year extension of stay. I have been in Thailand for 19 years and found out understanding the face saving rule saves a lot of headaches.