Thanks to all the input and I have ditched my Hiace camper van conversion idea.
https://aseannow.com/topic/1256625-foreigner-driving-a-toyota-hiace-in-thailand-and-border-crossing/
Instead, I am now keen to purchase a new (2023) Ford Ranger Wildtrak (4x4), which is just out in Thailand now, and maybe equip it to an overland type of vehicle. I contacted the local Ford dealership and they said if I book it now, they might have the vehicle ready by this September... quite a long wait for me because I don't have a car yet.
Would it worth to try contacting different Ford dealership in Thailand for availability (I am thinking some dealerships might be bigger with more quotas... is that the case in Thailand?) Would it be a lot of trouble down the road (servicing) if I don't buy it from my local dealership?