I am a PhD student from Ecuador studying Software Engineering. I received my US green card a couple of months ago, but I did my bachelor's degree in the US and applied to the PhD program as an international student, so hopefully my experience will help you.
Something that my professors told me while I was getting my bachelor's is to NEVER accept an offer that does not pay for my education and does not give me a monthly stipend for the entirety of the program. They told me that this is very common for PhD programs in the sciences. I know that this is true for Computer Science and Software Engineering; I am getting a monthly stipend and do not have to worry about any financial obligations with my university since it is all covered by the research money that my advisor has. In fact, I have no idea how expensive classes are here in my university since I do not have to worry about that. Now, I am not sure about Biostats or Statistics, but since it is a STEM field, the same cases might apply to those fields. You might be able to contact the business offices of the programs that you want to apply to check with them.
Another thing I was told when applying to graduate school is to apply for external funding such as fellowships or grants. This is beneficial for the university since they do not have to spend money in you and they get a graduate student who will do research and make them money. Unfortunately, all the fellowships and grants I looked required the applicant to be a US citizen. So that did not apply to me. Some universities that I applied to had scholarships for international student, although I did not get any of them.
You will have to pay for your own health insurance and living arrangements, but most universities will help you with those tasks once you have accepted their offer. All graduate students who I know can survive with the stipend they receive and the expenses that they have. I have never heard of a graduate student that is struggling with their finances.
In summary, being an international graduate student in the US is not that complicated as it seems. International students are actually the majority group in graduate school! If your program is in the sciences, you might not have to pay anything for the program and you will receive an stipend for your work. I have never heard of an international graduate student that left their program with any type of debt.