Get Medicare part B if you expect to return to the U.S., there are penalties for applying late. It will not, however, apply in Thailand. Not sure about SSI, check that, but getting your Social Security is fine.
probably two in one year... but, there could be push back after one time. It's up to the individual immigration officer. The law is still new and untested.
Da Nang would be my choice. Cebu is also quite ok, and everybody speaks English. Singapore for a more western, luxury experience. If you are a foodie, Da Nang or Singapore.