Ok, I just suggested a scooter as the ones in Thailand (eg. step-through 100-150cc autos/semi-autos) really are a great fit for 90% of the use-case riding done there. But, if planning on touring around: we did so for a week in the north 2 up on a Kawasaki KLX250 and it was perfect. More than enough power, long susp. smoothed the rough roads and no issues with rough dirt tracks. May be a bit tall (standover) for you though, depending on your size, though can be lowered a bit to fit. Safe travels!
Re: taking a course on a Harley... some jurisdictions don't require separate MC license/endorsement to operate very small displacement MCs (eg. scooters) so if this applies where you live and there's anywhere nearby that rents scooters (if a tourist area and the law is as I stated then somewhere likely will rent them) then just watch some youtube vids and rent a scooter and practice in quiet areas to start.