a) mail - how do you continue to safely get US mail overseas
Go electronic for as much as you can. Bank, Brokerage, accts, ...etc.
I am 90% there with my important stuff. I think I saw the usps offer forwarding for a price, but not sure if it sent to overseas locations... probably not. My snail mail goes to DS house while I'm away.
b) money - if you keep your money in US mutual funds, how do you access them? ... get Social Security checks .... pay taxes ... overseas?
I maintain a checking, savings, visa account at bank. I pay all bills electronically. The one or two I can't I have my DS pay for me.
Use ATMs if they are available in the country that you go to (some 3rd world countries don't have them yet
). Banks, especially in 3rd world are unreliable and/or expensive (exchange rates and fees will kill you).
Have your brokerage/MF firm set up ACH transfers, so you can request or setup monthly transfers to your checking acct (the one with ATM access). SS, when I apply for it, will go to my checking acct (ATM-able), I haven't figured out taxes, but I am out of the country for only part of the year and will return in time (this year) to file an extension and work on my taxes. I plan on figuring out e-tax for next year.
c) medical - how do you pay for medical? Use US medicade (or similar)
don't know have no need yet ... I come back each year for my physicals and dentist check up.
d) US citizenship - do you have to travel back to the US each year to maintain it?
Don't EVER give up US citizenship. If you are ever in a jam, you want to have entered a foreign country on your US passport ... otherwise, the US govt may not be able to get you out. As pointed out by others, it is harder than hell to get rid of you US citizenship ... they want their god given right to your taxes.
a hundred other things...
If you haven't tried living overseas before, you will definately be in for a culture shock. Little things, like waiting in line only to figure out no one else waits their turn, ... boiling all your water to drink/brush your teeth or having to buy lots of bottled water.
Bureaucrats that make ours in the US look like their your personal servant. Hot water and electricity available for the WHOLE DAY (and night). Services that are taken care of for you at home that you have to take a little extra effort at ... like taking out the garbage. OR You have to pay up and live in an expat community and pay for all the luxurys (read normal services you get in the US) of life.
Depending upon the country I could go on and on.
My reccomendation would be to take an extended 'visit', say two or three months, to a country you would like to live in. Rent a furnished condo and try it out for size. Learn to be conversant on the basics of the language, so you can get from point a to point b, buy things at the local market, ask for help, ask if they speak english ...
I think it's not for everyone, ... but if you are flexible (in thought and action), an adventurer/explorer, and love to experience different things, then you may be someone who would love to be one of the many expats out there.
Good luck.