Boomer2020

While abroad you can only sign up for parts A and B.    That cover is not so limited as it covers most expenses in hospital (after deductible) and 100% lab fees and 80% doctor.  Also it limits the bill that doctors charge.  

When you return to the United States you will be able to sign up for part D prescriptions.  

You can also at that time sign up for any  Medicare Advantage Programs that you want.  (I dont like these as original medicare is accepted my nearly every doctor.  HMOs are quite limited and PPOs can leave huge bills when outside plan. )  

To get a medigap plan you need to undergo medical underwriting if it is outside the time period for your signups.

I had about $10000 of doctor part 2 bills.  Medicare only allowed the doctors to Charge about $5000 and medicare paid $4000 and I would have paid $1000.  I have a supplemental plan G and it costs me $160 per month but it covered that $1000.

Note that the  supplemental plan I have covers me 80% for the first 60 days of travel outside the USA lifetime benefit max $50000.

Since you can enroll in Medicare Advantage in the first month or two you move back to the USA.

Manila US Embassy  is where I sent all my enrollment material for Parts A and B.

I got Part D immediately when I qualified but since I was 3 months past my enrollment window I have to  pay 3%more.  But you do qualify without underwriting when you return to USA must be done within 60 days I think.

I got a medigap policy at that time also.  I had to answer a lot of questions..  Which were reviewed  before approval.  Some companies have more questions  than others.   I used a broker who knew which companies to avoid etc.  

I think you should sign up for both A and B.   IF you plan on staying mostly in Thailand, don't get a supplemental plan.   The cost v amount of cover if you live overseas is not worth it in my opinion.   If you do plan on getting cover in medigap, do it before underwriting.  The best plan is now G.   If you get a deductible  G called a High G, you can save on monthly dues as   Regular G is about $200 month and High G might only be $60.   You would probably need a  US post address to maintain billing.  Etc.  This might affect taxes. 

Good luck.