If the US is anything like the Brits you will need a bulletproof reason for her having to return to Thailand. They don't mind her coming they just don't want her to vanish and stay. Probably 20k baht ($600) is the going rate nowadays but I would be asking about his success rate. Good luck.
Long term rental depends on 'how long'. The rule of thumb or guesstimate here is monthly rent is
*****
th of the property value. So if you are here for the long haul then buy. I know two people who have been here almost twenty years and only rented, and now have nothing to show for it.