Someone is telling you to trust them, when you have never met them. Trust does not work like that. The fact you are posting highlights your own doubts which you should take note of. You can apply yourself via a well-respected Muay Thai Gym experienced with DTV such as MuayThai Fever or Tiger Eye,
It seems best to get the DTV and return and makes better financial sense. If you do enter on a SETV make sure you have a ticket of departure, 20,000THB cash/currency equivalent and accommodation booked.