I suggest you attend the Seminar/open house organised by IDP. [http://www.singapore.idp.com/]

There are many case of medical students who had completed poly diploma, or 1st degree in Life science related courses, who then went on to work for a few years before they prepare and then take the "medical entrance" paper (I cannot remember the name). Working in the same manner as GMAT/GRE, there is a score which you can use to apply to medical school.

My close friend who recently did a total switch from field X to become a medical student in AU, told me the background of his batch are generally priviledged ones.

If to have a Dr in front of your name is what you are after, consider doing a PhD or something. Pay wise for an average Medical doctor is not exactly fantastic (unless in the days of yore), relative to the long work hours, constant need to earn Professional Units credits and medical insurance, and looming legal suits.

The career path for a locum (part-timer) is patchy. To open your own clinic (the ultimate aim), it's tougher than before. Inventory checking, more audits, manpower, rental, etc.

Be careful what you wish for. [Talk to someone who had been there and done that. Similar background as yours. Where are they now and what's their critical success factor?]