If he just comes back to Thailand he will get 45 days to sort out the visa process without worrying about the medical insurance and he can get something done once he is here
You can rent the land for 30 years or even 60 years
You cannot use it for any hotel of any type
You have to have a work visa just for a start then you will have to get a permit to build your hotel and that will depend if your allowed to build it in that area plus have some thai people working for you
You can Google most of the information you need
But your best thing is come to see a lawyer in Thailand for the help you need