It is most important to have this document
It is perfectly legal in Thailand to have a Living Will. However you need to be aware that it is a directive that you set up before hand to tell the doctors, hospital, staff NOT TO keep you alive on a machine, NOT TO force feed you, etc. It is not to tell the hospital that you are sick of living and want them to terminate your life by drugs or another method.
There are 2 legal conditions that activate the LW
You must be in a terminal phase of illness ie cancer and dying or brain dead after a stroke or accident. Health Act part 12.
The person you had sign your Will as the Surrogate must meet with the hospital and staff and show them the legal signed Living Will otherwise if no Will then you could remain on life support for a long time and this is expensive.
Spend small money to make a document to stop paying big money strapped to a machine later.
Read this artical from the Bangkok Post. The old age care home is something else and I know of a few around Pattaya and Hua Hin.
A stay in hospital could run at 70,000 baht per day and this will soon deplete your savings. This is not to say you should not be in hospital seeking treatment, however you might get the same treatment at another hospital much cheaper. But without having a directive of some kind there is no way your proxy can have you moved to a cheaper hospital. You are powerless.
Due to my business I cannot be in this state and so have personally left a Last and Living Will. Perhaps you should consider the worst case scenario to prepare for (and hope it never happens) but still you have the docs in place if needed. firstname.lastname@example.org