I was told that these are not even legal despite there being many 1,000s on the road. When the bike needs a test they have to remove the sidecar and test it as a stand alone bike. probably most aren't even tested
Oh and if you were going to get a visa exempt stamp and a 30 day extention you will still be over the 90 days based on your flight so will need to leave Thailand anyway.
I'm sure if you have a flight out of Thailand within 60 days you will be okay when you check in at the airport. Get a cheap flight to Vietnam as its a great place to visit anyway. Phone them up but I'm sure you will be okay as it's on the UK.Giv website you don't need a visa
I'm surprised you have spent hundreds of hours researching Thai laws and still asking if you can import cars. Facebook will give you not hundreds but thousands of answers to your questions with so many having mates who have done everything. You must know by now then that in Thailand if you go to 2 different immigration officers you will get 2 different answers. Good luck, you never know unless you try. I would also factor in that if you did bring all your stuff here it's just as hard to take it home should you change your plans.
Basically forget importing cars it's not allowed anymore. Forget importing personal items as they will be taxed ar a ridiculously high rate irrelevant of their true value. You can own a condo but not land. You can lease land under the guise of a company but it sounds like you haven't visited Thailand so why not just come for a few months and experience some of the challenges of staying as a tourist let alone starting a business.