Anyone can buy a vehicle in Thailand, there are no visa or residency requirements. The DLT requires foreigners to present their passport and a certificate of residency, workpermit, or yellow Tabaan book to register a vehicle in their name (car or motorbike).
Depends on the item. Customs can open anything they want. If it's not a lot of stuff I'd error on the side of caution and just bring the wrapping paper with you.
It's not that you cannot transfer more than 50k, it's that more than 50k baht requires the receiving Thai bank to make a specific filing with the government, and apparently not all banks are setup to submit those filings.
I haven't seen any issues with anyone transferring to a Bangkok Bank account, other banks some people's seem to go through, others there's a delay. Wise doesn't seem to be affect as much as the other transfer services.