To answer your question SS and other Federal pensions are not subject to Thai tax according to the Double Tax Agreement. Everything else if brought into Thailand would be taxed.
Never been in one but I have seen many on the road. They are the usual typical Thai driver. If there is a 2-3 meter space between two cars and they want to pass they will squeeze in at a high rate of speed and pass into the next lane. Overall Thai drivers really suck. 5555