What would you say are the most important financial concepts one should completely understand before starting their business?
Finance Financial Entrepreneurship
I'd say that knowledge of break-even analysis would definitely help.
If you plan on raising money someday, get an accountant from the get-go. Otherwise, you'll beat yourself for not doing so when the time comes to raise money.
RockHealth has a Startup Accounting 101 post that's also a helpful read.
1) Bringing in more money than you spend is generally considered "a good thing".
2) Hire an accountant.