Re Doctors Without Borders:
--I don't think you need a BSN but you will definitely need nursing experience. Surgery, ER, OB/GYN, Peds are a few useful specialties that come to mind, though people with lots of different backgrounds go. They have a huge range of projects including primary health, inpatient, surgery, nutrition, vaccination, community health, L&D, HIV, TB.... the list goes on.
--Personal travel & international experience can be a big plus with them, even if it's not specifically medical - they ask about this in the application/interview.
--In my experience, knowing other languages is never wasted, but Spanish assignments with Doctors Without Borders are few and far between. French, however, is super useful with them. They will sometimes send you to French immersion once you have worked with them for a while, but if you have languages to begin with it will help your application a lot.
--The organization is non-religious and non-political. I suggest making sure that you feel aligned philosophically with the organization that you choose. While missionary nursing and working for non-religious humanitarian orgs share a lot, there can also be significant differences in everything from what they are looking for in candidates to the approach to work in the field.
--Doctors Without Borders tends to be more clinical than a lot of other international health orgs but as a nurse you typically would be in a management role. Sometimes you do more direct clinical work, but as a rule you should be prepared to manage people and projects. Of course you still need to know your stuff clinically in order to manage/teach/etc.
--I think they will help arrange loan deferment but the actual pay you'll get will not be much. If you want advice - I say get a job in a useful specialty, throw as much as you possibly can at those loans for the next couple of years, maybe try to do a couple short-term things in the meantime for a few weeks at a time to keep yourself motivated, etc., and gain more experience.
--MPH - could be a great choice for you. RN/MPH is very marketable in the humanitarian field. BUT could add so much to your loan burden that you would feel too limited to do low-paying work. Timing is essential...
--Great advice above from Cheryl. Important to be resourceful, manage stress, be flexible. You will end up using everything you learn.
Happy to answer any other questions.. good luck to you.