Hey iMSPH-
The Columbia Postbac curriculum is relatively structured and ensures that you fulfill all the requirements for medical/dental school. You are given clear guidance on what courses to take and in which order. You will get the courses you need from top notch faculty shared with Columbia College (our classes are academically integrated with undergraduates at Columbia, but administratively separate). However, there is also some flexibility, given your situation. For instance, you can talk with your advisor about taking higher level science courses if you have finished program requirements. Also, there are pre- physics, chemistry, and calculus options if you come in feeling unprepared for college level courses (a placement test will help guide these decisions). Additionally, students are required to complete 120 hours of clinical and research experience which, in my experience, has been incredibly motivating and an integral part of the curriculum.
Here is some information on the curriculum from the Columbia website:
http://www.gs.columbia.edu/postbac/curriculum-and-courses
As to your second question- living in New York is expensive, for sure. I know that people in the program make it work in a few different ways. Some are working (either full-time or part-time) and take classes around their schedule. It is possible to complete the Postbac program through a part-time sequence. Here is the information from the website:
http://www.gs.columbia.edu/postbac/part-time-program-sequence
I also know students who work 3 days a week and take all their classes on the other two days. I know some people who have a working spouse/partner that help provide financial support. Other students (including myself) planned on entering the program and saved money from years of working before starting to take courses. A final option that a lot of students choose is to take out private loans for living expenses. I try to think of it as an investment in my future. Also, I have always wanted to live in New York and there are lots of ways to have fun here without spending too much.
Let me know if you have any other questions!
-kf